https://www.uhaul.com/Tips/Moving/About-U-Haul-Roadside-Assistance-20869/

If you can’t get it back on track yourself, or you aren’t confident you can do it without damaging your truck, check that out

Assuming that pushing it up and pulling it back down doesn’t work, and the track is clear of obstructions…

There’s a help (roadside assistance?) phone number in the little receipt packet they gave you.

The more time you spend screwing around trying to fix that door yourself, the more likely it somehow comes back to bite you in the ass.

Drive to a UHaul and ask them for a new truck.