One side has skipped a tooth. I did what the owner's manual says - remove the stop-screw at the bottom right of the frame (the long bolt that hits the up-down switch to keep the carriage from going off the rails). If possible, get a helper.
Make sure the frame/baseboard is level - shim it if it isn't. Hold the bottom tube of the carriage with both hands, and have someone work the switch so the carriage drops from the tracks - just the bottom tube needs to drop free. Now lift the carriage up so it contacts the teeth left and right, and have someone flip the switch "up". Push the carriage up so it catches and begins to rise. Check it for level - it should be about the same level as the baseboard and frame-top. If not, try again. Might take a few tries, but you'll get it. If you can balance or tape a level to the top carriage tube, it can help you get it lines up before someone hits the "up" switch.
You can do this yourself, but then you need to lift the carriage with one hand and hit the switch with another... or grow a third arm!