Stopping down to f8 is not going to give you sharp corners w/ a medium format camera. They need to be stopped down further than that.
Before you disassemble the camera as suggested above, ck to see if the rear element has become loose as well. Generally, lens elements on TLRs do not loosen up on their own, so it may be that someone had the front element out to clean the shutter blades or something and did not get it screwed in tightly when putting it back in.
What you need to do is ck the focus w/ a loupe as suggested in the post above. You will need a tripod, a release cable to hold the shutter open on 'B' (on some models), a loupe or a 35mm slr lens to use as a loupe, and a piece of ground glass to lay on the film rails ground side facing the lens. Scotch tape stretched across there will do in a pinch, but the ground glass is much easier to work with and gives more accurate results.
After you have all that, put the camera on the tripod facing something like a tree or pole at least 100' -150' away w/ the camera set to infinity. If the image on the GG is not sharp, ck the image on top to see if it is sharp on the focus screen. My earlier model Rolleicords had an infinity stop screw on the focusing knob. You need to pop the cover off to see it. That allows you to adjust the taking lens infinity to the same image as is on the focus screen.
Also, ck to make sure that the focus screen is properly seated into position and that the focusing lens is not loose. Both it and the taking lens need to be in agreement. TLR's are simple box cameras w/ two lenses, and if you are at all handy, patient and thorough, you can usually figure things out and get them in the zone. The more complicated cameras like the Rolleiflex models are a PITA to work on because of their complicated controls for the film winding and aperture and speed settings.