agree with ground cable chk - might also chk for main fuze blown... (if so, look for cause)
Might also try jumping engine from another battery - those small mower batteries are somewhat notorious for crapping out with dead cells, etc whilst showing 12 volts at posts - if engine cranks with jumper cables and different battery, pull tractor batt - take to qualified shop for LOAD test to determine usable condition - if tractor batt OK, then time to trace starting circuit including safety switches for glitches.
