Database upgrades can fail for 2 reasons:
1. The database cannot be found because it is in a non-standard location that MediaMonkey could not find.
To resolve this update MediaMonkey.ini with the path using DBName.
Or if you're upgrading from MediaMonkey 2.x run, MediaMonkey from the command line as follows:
MediaMonkey.exe /OLDMM2DB [path to old database]
"C:\Program Files\MediaMonkey.exe" /OLDMM2DB d:\MediaDatabases\MediaMonkey2\MediaMonkey.mdb
note: "" are required for paths containing a space
This will cause MediaMonkey to detect and upgrade the old database when it runs (note: if you're not prompted to upgrade the database, it is either because the old database cannot be found OR because a MediaMonkey 3 database already exists (in which case you should delete it).
2. A database failure has occurred during the upgrade process. Sometimes this can be resolved by installing the previous version of MediaMonkey
which was installed (http://www.mediamonkey.com/support/index.php?/Knowledgebase/List/Index/16/software). After installation, open MediaMonkey and go to File > Maintain library. In the window that appears, check "Optimize database (complete)". After the process completes, reinstall the latest version. If this fails, your database may have become corrupt.
For causes and solutions to such corruption, see:
Error: Database Image is Malformed