The Fail indication usually is an indication of a large power abnormality (beyond the capabilities of the unit), an internal error or some combination of both.
To clear the Fail error state, power must be cycled as you have done. In the meantime, whatever may have caused the fault might have been cleared. When the power is then cycled, the unit may turn on without any indication that a fault had occurred, in other words the fault had been cleared and the unit is ready for normal operation.
When a fault has occurred and the display shows FAIL, the error code can normally be retrieved by pressing and releasing the OFF button. With this additional information we might be able to figure out more details of what precisely might have happened to these particular units.
Unfortunately we probably cannot determine the exact cause if the error is cleared. If the issue happens again please try to obtain additional error information by cycling the OFF button prior to cycling the source power as described above. If possible please keep track of which units and location experienced this problem so that if it would happen to reoccur we would know that it was the same units and/or location.