See if the index is up the creek. Right clickon 'Sent' folder in folder pane and select 'Properties' clickon 'Repair Folder' click on OK

select another folder and then reselect 'Sent'.

Are you able to read emails ?

re : Is there a "sent" file I can copy from the first pc and just bring over those sent files again with content?

Remember to exit Thunderbird on both computers before doing this. Default location of profiles: POP account: C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\Mail\pop account name folder

In pop mail account folder you should see a list of files that have same name as folders. mbox files do not have an extension and they contain emails. You can copy and paste these files between the two profiles to get copy onto other pc.

See image as guide.

I have found all the prior "sent" emails in a folder "sent.mozmsgs" for that pop account. I copies these sent.mozmsgs over, but when I open Thunderbird and look in my "sent" folder they do not appear.

The mozmsgs files are used for search indexing and don't contain the message contents. Your best option is to copy the mbox file named Sent or Sent Items from the account subfolder to Mail/Local Folders in the current profile, as explained above.

Thank you to everyone. I copied the entire pop email folder over and that included the inbox as well as the sent folders and all is working fine now.