there are long worm gears to move it forward/backwards. Its like a big drill bit that goes through a bit that moves the track. The are greased from the factory and over time will build up dust on them that gets into a bind. If you can get them out, take them out and give the long drill-bit-lookin piece a good soaking of wd40 to clean them. Wipe it down and add a light coat of lithium grease and I am willing to bet they will start working again. Mine locked up years ago(7-8 I think) and I did this to it and have not had an issue since.
The other possibility is that the piece that the bits go through has broken. I remember somewhere reading a DIY repair for that fix but it was ages ago. Some light searching could probably dig it up.