That happened one time on our trailer while on an overnight traveling to Florida for the winter. Got some help the next morning pushing the slide back in and left it in the rest of the way. Once we go to our winter site in Florida I called a mobile tech. He went under our trailer and came back out about 30 seconds or one minute later and the slide was fixed! :S
Seems there is a set screw, that holds the shaft driving the slide, in a collar. The set screw loosened up and let the shaft slip out of the collar. The Tech simply slid it back in and tightened up the screw. It has held now for over 12 years. :)
Hope yours is as simple as that.
Barney