Are these jacks hydraulic? (Sounds like they are) if so the valve may have failed one of two ways..
Find the vlaves take a piece of feromagnatic metal (Flat not edged) (Means magnets stick) and while partner attempts to lower the jack from control panel hold the thing near the coils.. If coil working the metal will snap to as it were (I like rubber covered metal.. Dip it in Flex Coat)
NOTE to extend jacks ONE valve per jack on some RV's on others front jacks have one valve between 'em.
if the magnet test is good the valve may have an issue.