That happened to my previous 1996 Force 120 - after I changed its lower unit impeller gear. To resolve, I had my wife climb into the boat and as I "gently" tried to push its lower end upwards, she took its shifter control handle and moved its back and forth very slowly. Must move back and forth with its gears engage (re: Not only moving its throttle position). When she moved the control handle back and forth 3" (or so), its upper gear aligned up perfectly. Lower unit slipped back on (upwards) like a grease inner rubber glove. Remember to gently push upwards as re-aligned of upper spindle gears is done. Brute force will break something (that isn't properly lined up).
Hope this helps.