First, try closing out the project and quitting greenfoot. Then re-open greenfoot and the project and try again.
If that does not work, see if you can open the .java file with a text editor? It is located in your project folder. If so, save the text in a separate file; then see if greenfoot will allow you to remove the actor class. If successful, create a new class and copy/paste the code from the file back into the new class.
Hope this helps.
Thanks, turns out the .java files were gone, only the .class files were there. I ran the .class files through a decompiler and put the .java files back in. It works now, although I have no clue what happened to it in the first place.