Try checking all the things Gary mentions first. Also make sure your shift lever is firmly in park. Jiggle the lever a bit to make sure.
If none of the above fixes works, our motorhome also has a relay that interlocks the slides with the ignition switch so the slides can't be activated while driving. If you have a similar relay, the relay could be bad or it could be stuck in the open position. That happened to us twice; and when it did, I rapped on the relay with a screwdriver handle to unstick it. I can't tell you if you have a relay or not and where it is, but that could be your problem.
Good luck, and let us know what you find.