This is almost always a result of a modded file in the admin folder being incompatible with PHP v5. I would guess that your host has just upgraded to v5. Rename your admin folder, reupload a fresh unmodded folder, then move or replace the files from you old admin folder into the new one one at a time until you find the culprit.
Or your "playing around" involved changing or adding a file in the admin folder.
good and VERY
inexpensive hosting CLICK HERE
I can stop all your spam. I can upgrade or update your Board.
PM or email me. (Paid support)