The first thing to look at is, is it getting 12v ? I had a friend that had the same complaint... Went over to his house and found that when he put his battery back in, he forgot the wire for the power jack
Power jacks are usually a direct connection to the battery by a separate fused wire. Found the wire where he had tucked it out of the way, reconnected it and he was good to go... We then had a couple of beers, a good laugh and told the wives that it was one of those "really bizarre electrical problems"... Assuming that you hooked it back up when you put the battery back in...:
Time to get out the trusty VOM
Quite often, with those tongue jacks, the fuse assembly gets moisture inside and the first thing you know is corrosion settles in around the contacts. Dissimilar metals carrying voltage with a fair amount of amperage (as in a tongue jack) will generate a lot of corrosion rather quickly. Pull the fuse, check the contacts and then start tracing the flow of current. Most come with those silly old glass fuses and the crummy twist together holders. I usually, on a new install or repair, I cut that out and install a good bladed, auto resetting fuse, copper grease on the blades and then wrap the whole thing well in fusible electrical tape.
If you are getting 12v into the fuse, and after checking the "out" side of the fuse, you are getting 12v out, the next thing to look at would be the ground path. Power jacks don't have a ground wire (well, most don't, I'm sure there are exceptions) and they rely on the mounting point for a ground to the trailer's frame. Whenever I help someone install a power jack or walk them through it, I make sure that I sand paint off of the jack's mounting plate and the area that the plate covers on the tongue and then use a paste grease that has copper in it (it is actually an anti-sieze grease, but, the copper has excellent current passing properties and prevents corrosion of the mounting surfaces that were sanded to bare metal) to coat both surfaces. It is possible that your mounting points are still painted and the bolts, particularly if they aren't installed on toothed washers aren't making a good ground path. The ground path is the most often overlooked part of a power jack installation, both by the factory and DIYs...these jacks use a lot of current and they can't draw that current in if they can't pass it out.
The switch is another problem area...again moisture gets in them (they aren't the high tech, expensive marine rated versions) and they fail. That's why most of us keep the jack's head covered with a bucket, or in my case, a water proof bag. The water proof bag was my choice; it kept the head dry, I didn't have to take it off to work the jack's switch and once tied on (it had a draw string closure) it wouldn't come off going down the road. It is possible the switch has failed. If you've established 12v into the motor and the ground path is clean and solid, the switch may be your problem.
It is possible the motor's fried, but, my money is on the current's path. In my years of using them, I've never had a motor go bad...wiring (current path) a definite yes, gearboxes yes (I've seen one), but, never a motor, so, IMHO, that's a long shot. Usually a mechanical reason (motor or gearbox) will give you "advance" warning; a grinding or ratcheting sound or a gradual reduction in ability to lift.
Power jacks are a simple device and 99% of the problems with them are usually with the electrical path. Dust off the VOM and good luck!