What do I do to get my database working again?
Sounds like you have a corrupt .tcd file or one of the songs you added has metadata that is flipping the database out, have a look at your tracks with an ID3 tag editor for any tags that look suspicious/wrong and or have a look in your .rockbox folder and remove all files with the extension .tcd
This is a way to force the database into rebuilding itself, however after you have removed the .tcd files and while your device is still connected to your PC it would be wise to check the device for FS corruption.
As you recently added music, and the database broke I would focus my attention on metadata of the newly added tracks before deciding to remove all .tcd files...it seems likely one of the new tracks is causing it.
After you've removed the .tcd files, and checked for file system corruption reboot the device and simply select the databse from the main menu and it will prompt you to initialise the database again.
Hopefully all goes well this time.
[St.]