This could be an anti-virus software that has recently updated its definitions etc.
However, here's what I would do:
If it works at all, launch MC20 and make a backup of your library file immediately. Put it on the desktop, or anywhere other than the default location.
Then uninstall MC20.
1) Don't delete the registry entries etc. Keep the library files too.
2) Reinstall the latest version of MC20.
3) Restore your library from the backup. Don't restore your settings for now.
If that doesn't work:
1) Uninstall MC 20, but this time delete the registry settings too. Keep the library files.
2) Reinstall the latest version of MC20.
3) Restore your library from the backup. Don't restore your settings for now.
If either of those approaches works, you can then attempt to restore your previous settings from the library backup.
If you cannot even get into MC, don't worry, MC makes regular backups of your library. Locate these. Copy them somewhere handy, and do the above, but use a backup that was made by MC to restore to.
Ask if you are not sure of anything.