what's inside the subdirectories named as accounts?
In each account subdir there should be subdirs named like folders defined in the account. In each such folder, there should be 2 files named messages.tbb and messages.tbi (or .ebb and .ebi, if you used encryption)
Is it so ?
Does the registry path match the main folder path?
Do you see old accounts or not?
If not, try creating a new account named as one of the old ones
Are encryption settings now the same as before?
If you used encryption before, you should use it now as well, with the same password.
If you didn't use encryption before, you shouldn't use it now either.
AFAIK encryption usage can only be defined at install time and can't be changed afterwards.
To switch encryption<-->no encryption, you should uninstall theBat, delete the above registry key, and install again specifying the other setting.