A lock mechanism having a mechanical combination lock and an electronic lock, wherein the mechanical combination lock serves for effecting a fail safe entry in case of failure of the electronic lock. More specifically, the lock mechanism includes a mechanical combination lock having a bolt-retracting linkage (145) which is tripped by inward movement of a dial into a first operative relationship with a driver on the drive spindle. An electronic latch prevents bolt retraction by this bolt-retracting linkage when in the first operative position until the electronic latch has been operated. However, when the tumblers of the lock have been aligned and a gate bar enters the tumblers, the bolt-retraction linkage is caused to assume a second operative relationship with the driver so as to permit bolt retraction despite the latching effect of the electronic lock.