This is a known issue with the EF 50mm f/1.4. The most common cause of the problem is that the guide slots at the end of the focus collar inside the lens have become bent. The most likely cause for the bent collar is an impact to the lens, such as being dropped, when the focus is set to a short distance. Always storing your EF 50mm f/1.4 with the focus set to infinity will help protect your lens from this problem if it experiences a sudden impact. The screws that retain the plastic guides inside the slots on the helical collar can also work loose and cause similar focusing problems. The exact same dis-assembly/assembly procedure is required to get to either problem to repair them.
There are some online resources that show you how to repair the lens yourself. Do so at your own risk, but it is not too difficult a repair as far as lenses go and if you have an aptitude for working on small mechanical devices and can follow long, detailed instructions carefully you can probably pull it off.
Good Luck!