I think your file system got corrupted. Anyways, usually a copy of it can be found in c:\windows\system32\dllcache (a copy from c:\windows\system32) or c:\windows\i386 or c:\windows\$uninstall that is a folders where updates saves copies. If found then copy to c:\windows\system32.
rundll32.dll is essential windows process. If you don't find it then you have to reinstall/clean install OS. Good luck.