That's usually an indication of gear failure. May have teeth broken off and causing the two to lock up. Or the broken tooth is getting picked up and wedging between the two gears.
But with no more information than you've given it's hard to diagnose.
Maybe a description of what you were doing when it broke?? To get it unlocked do you simply backup?? What type front axle assembly does this tractor have?? When it locks up does it try to slide one front tire or do both stop turning?? Do you hear any crunching or grinding noises when it does move?? Or does it never move??