Apparently Diskeeper caused some corruption of TB's files, that implies that not everything might be retrievable.

You mentioned that you've got a three month old backup, that's a nice start.

Rename your mail directory. (Your mail directory is the diretory with subdirectories for every account.)
Uninstall TB.
Install TB again
When you were using OTFE (On The Fly Encryption: encrypted message base) please enable OTFE again with the same master password as before (that's really important), when you weren't using OTFE don't do that now.
Run TB
Restore your backup, but don't start collecting mail.
Everything ok?
Close TB
Make a copy of your full mail directory to a safe location.
Now comes the testing and trying:

Copy an account from your old mail directory on top of the same account in the restored mail directory.
Start TB, but don't collect new mail, check whether your updated mail account is working as intended, if not then close TB and copy the account again from the backup you made before the testing started.

Follow this procedure for all of your accounts and for all of your common folders. When you're lucky it wasn't the message base that went bad, but something else.
After following this procedure please report back with the follwing data:
were you using OTFE
did you retrieve all of your accounts
were you using common folders
were you using an anti-spam plugin (if so you might need to train that again) and which
were you using an anti-virus plugin (and which)
what version of TB are you using