You're probably missing files in your /includes/modules/payment folder and/or your /includes/languages/YOURLANGUAGE/modules/payment folder.
Or, maybe you have "extra" files in those folders ... sometimes people mistakenly make backup files by renaming files but keeping the .php extension ... which can cause problems.

Check those.

Then check your server's errorlog for messages.

Then sit down and figure out what you've changed on the server recently.