Minivan runs, but gears won't engage


2000 Mercury Montana - runs fine. Yesterday, gears just stopped working while we were driving down the road - the engine just revs, as if it’s in neutral. We had no warning signs of skipping gears, or anything like that. Seems like something just came loose, or is it electronic? Is it something I can fix myself (I can’t do major stuff - I can do brakes, alternators, tie-rod ends, etc.) SA