Suddenly I have low voltage at the wire that feeds the garage?

I have a No. 10 wire going underground to the garage. It's only for a light and an outlet. It's worked fine for years but suddenly the elec. quit. I found the junction box where the UF wire comes up and connects the garage. I separated the feed wire from the wire that goes on to the garage and only have 50 volts. It's only a two wire (no ground) but has always been fine. What could be going on? I could take my VOM and just stab the ground probe in the earth (with the other probe on the hot wire naturally) and get 100 volts at least. Any help would be appreciated. Thanks Mike