MAINT. CREW WAS REPAIRING A CEDAR RAPIDS CONE CRUSHER. TOP OF CRUSHER HAD BEEN REMOVED EXPOSING A HOLE IN THE WORK DECK, WHERE THE CRUSHER ACCUM. HAD BEEN. THE INJURED PERSON THOUGHT SHE WAS S TEPPING INTO THE SMALL OPEN HOLE & OVER CORRECTED HER POSITION BY REACHING FOR A NEARBY LADDER. A FEW HOURS LATER SHE COMPLAINED OF A BACK PAIN & AT THAT TIME WAS TAKEN TO A MEDICAL FACILITY.