Theres an external limit switch open, or the board has gone past the maximum number of retry/recycle attemps to establish flame or not lose flame after a certain time.
After an hour in external lockout the control should automatically attempt trial for ignition again. Cycling power restarts all of these counters and timers but of course does not resolve the root problem and can cause even more issues.
You may want to just get a tech involved now, be safe with gas and don't waste a lot of time changing parts needlessly.....
Sorry, also meant to add it certainly would be worthwhile to inspect and clean the flame sense probe if this unit has been in for awhile, some fine steel wool or a scotchbrite pad will do.
Last edited by Done That; 11-14-2012 at 03:05 PM.