Set up the camera on a tripod, shoot the lens wide open, clearly focused on something within the close focusing range of the camera, and that's identifiable and distinctive.
Then stop down one stop at a time until you reach the smallest aperture.
Repeat with a different lens.
You could also note exactly the focusing distance indicated on the lens, and compare it to what you get with a measuring tape (measure from the film plane to the point of focus you see in the viewfinder).
I can't tell you what's wrong, but you should try to repeat what you did to see if it will happen again. If there is something wrong with the camera, it's important to be able to tell exactly when it happens, so that the repairman stands a good chance at diagnosing and fixing the problem.