I've never known of any keyboard procedure that could wipe the ladder. It is possible, however, to wipe the parameters, including the option bit that allows the PMC to be turned on. Are you sure that this isn't a case of just wiping the parameters to default?
If you think that might be the case, check the option parameters (they're the ones shown on the data sheet). If necessary, hand load these first.
Uptime Electronics, Inc.