Can you post your Troubleshooting Information? Help (Alt-H) - Troubleshooting Information

And there is no recent back-up.

Why not? It's about time to create one.

I know about the back-up...now.

And you mean this information below??

 Application Basics

   Name: Thunderbird
   Version: 31.3.0
   User Agent: Mozilla/5.0 (Windows NT 6.0; rv:31.0) Gecko/20100101 Thunderbird/31.3.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20141127140543
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) IMAP.Leidenuniv.nl:993, SSL, passwordCleartext
     OUTGOING: mail.euronet.nl, plain, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account3:
     INCOMING: account3, , (pop3) mail.euronet.nl:110, plain, passwordCleartext
     OUTGOING: SMTP.euronet.nl, plain, passwordCleartext, true

 Crash Reports

 Extensions
   CompactHeader, 2.0.7, true, {58D4392A-842E-11DE-B51A-C7B855D89593}
   Lightning, 3.3.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Test Pilot for Thunderbird, 1.3.9, true, tbtestpilot@labs.mozilla.com
   Lightning Calendar Tabs, 1.3, false, lightningcalendartabs@jlx.84

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     browser.cache.disk.smart_size_cached_value: 358400
     extensions.lastAppVersion: 31.3.0
     font.internaluseonly.changed: false
     font.name.monospace.el: Consolas
     font.name.monospace.tr: Consolas
     font.name.monospace.x-baltic: Consolas
     font.name.monospace.x-central-euro: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.tr: Calibri
     font.name.sans-serif.x-baltic: Calibri
     font.name.sans-serif.x-central-euro: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: Cambria
     font.name.serif.tr: Cambria
     font.name.serif.x-baltic: Cambria
     font.name.serif.x-central-euro: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 14
     font.size.fixed.tr: 14
     font.size.fixed.x-baltic: 14
     font.size.fixed.x-central-euro: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.tr: 17
     font.size.variable.x-baltic: 17
     font.size.variable.x-central-euro: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 17
     gfx.blacklist.suggested-driver-version: 257.21
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: af6232d8-04ef-4102-b304-d7fe3878cbd
     network.cookie.prefsMigrated: true
     places.database.lastMaintenance: 1419004293
     places.history.expiration.transient_current_max_pages: 53527
     places.history.expiration.transient_optimal_database_size: 85642444
     plugin.importedState: true
     plugin.state.java: 0
     plugins.update.notifyUser: true

 Graphics

     Adapter Description: NVIDIA GeForce 9300M GS
     Vendor ID: 0x10de
     Device ID: 0x06e9
     Adapter RAM: 256
     Adapter Drivers: nvd3dum nvwgf2um
     Driver Version: 7.15.11.7624
     Driver Date: 7-26-2008
     Direct2D Enabled: Blocked for your graphics driver version. Try updating your graphics driver to version 257.21 or newer.
     DirectWrite Enabled: false (7.0.6002.23200)
     ClearType Parameters: ClearType parameters not found
     WebGL Renderer: Blocked for your graphics driver version. Try updating your graphics driver to version 257.21 or newer.
     GPU Accelerated Windows: 0. Blocked for your graphics driver version. Try updating your graphics driver to version 257.21 or newer.

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: cairo

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.6
     4.10.6

     NSS
     3.16.2.3 Basic ECC
     3.16.2.3 Basic ECC

     NSS Util
     3.16.2.3
     3.16.2.3

     NSS SSL
     3.16.2.3 Basic ECC
     3.16.2.3 Basic ECC

     NSS S/MIME
     3.16.2.3 Basic ECC
     3.16.2.3 Basic ECC

Lightning 3.3.1 is the correct version for Thunderbird 31.3. And according to your Troubleshooting Information it is enabled. You do have an extension Lightning Calendar Tabs installed, which seems to be disabled. If you don't use it, I'd get rid of it. For the update from TB 31.2.0 to 31.3.0 the Lightning extension hasn't changed. I therefore doubt your problem has got anything to fo with the update (assuming you did update from TB 31.2.0.). What kind of calendar do you have - local or network? If network - is a a Google Calendar? If Goggle Calendar - how do you access it? Via Provider extension or CalDAV?

It is a local calendar, all the information is in lightning. I don't use google-caledar, maybe from now on....

After deleting the tabs and restarting several times, my calendar information reappeared magically. So I immediately made a back-up and now looking for a way to store my calendar in multiple places. Thank you for helping me.

As your immediate problem has been resolved, please mark this thread as 'Solved'. For any additional questions please raise a new thread. Thank you.