If your PC was hacked and the sent folder was for a POP account, they are likely gone, unless you saved on server. if the hacker went for the online account, they may all be gone, unless email provider can restore a backup. Thunderbird, itself, cannot assist in this.

I have been wrestling with a problem that sounds partly like yours. My computer was accessed from Nigeria. All my years of previously sent Thunderbird email disappeared. Also, a very large number of new innocent incoming emails were tagged by Thunderbird as suspect spam. A password change and a full computer scan over a week ago found some "virus" items and removed them. Problems remained, however. Yesterday I launched a full computer scan by Bit Defender (which took quite a while). BD found a trojan horse called TrojanGenericKD.72198276 and BD said it removed it. It also found two infected email items it could not remove that were in Thunderbird email archives. Bit defender gave the option of removing them manually -- good luck with my faded command prompt skills. The automatic removal option in the options panel scared me, as it was unclear HOW MUCH stuff would be removed. And it was unclear what the other "archiving" option would do. I used screen commands to see that the identified location, which was: C:\users\Dwight\appData\Roaming\Thunderbird\profiles\ [mostly hidden directories] The last directory had only two small files:96x914ek.default-release and a similarly named one In spite of the great uncertainty, I took the chance that BitDefender would only remove those two files, so I told YES for BD to fix it. BD said it had fixed it. I rebooted the computer, and all the old sent email had reappeared. Not sure everything is fixed, but it looks hopeful. The spaghetti monster fingers are still crossed. Good luck. Local Thunderbird experts are hard to find.