Repairing DatabasesKeePass can repair corrupted databases in some cases.
KeePass has quite some features to avoid database file corruption (transacted database writing, device buffer flushing, ...). However, data corruption can still be caused by other programs, the system or broken storage devices (note that KeePass by default is verifying the integrity of database files immediately after writing them, i.e. at this point of time, KeePass guarantees file integrity; however, KeePass of course can't do anything when the data becomes corrupted/unreadable at a later point of time).
In these cases, the database repair functionality might help you. Here, KeePass tries to read as much data as possible from the corrupted file, without any integrity checks.In KeePass 1.x, the repair functionality can be found in 'Tools' -> 'Repair KeePass Database File...'.
Anyway, if you've lost the master key for the database, the repair functionality cannot help you. Also, if the header of the database (first few bytes) is corrupted, you're out of luck too: the repair functionality won't be able to restore any entries (because the header contains information required to decrypt the database).
The repair functionality should be seen as last hope. Regularly making backups of your databases is much better and has to be preferred. Backups have no cryptographic security implications. There are plugins that automate the backup process, see the KeePass plugins page.