One of the really nice features of the Caleffi ZSR and ZVR Zone Relay Controls is the resettable fuses for the 24 V circuits. When a resettable fuse trips, it is not "blown" and does not need to be replaced, it just opens. If any of the 24 V circuits (thermostat wiring or zone valve wiring) experience a short or exceed the 40 VA rating of the transformer, the resettable fuse will open the 24 V circuit to protect the transformer. When this happens, the board will still have line voltage, but no 24 V (just like blowing a glass fuse). The relay board is fine, but there is a field wiring problem that needs to be addressed and fixed. Once the resettable fuse has cooled down (line voltage removed for a short time), it will close again.
If this happens, disconnect the line voltage from the relay board. Check out all the 24 V wiring to make sure there are no shorts. Make sure the sum of all the loads does not exceed the VA rating of the transformer(s). If all that looks OK, disconnect all the field wiring and then re-apply line voltage to the board. Check to see that you have 24 V at the R-C thermostat terminals, making sure the transformer and resettable fuses are OK. Power down again. One-by-one, reconnect the 24 V circuits and power-up the line voltage. This process should help to isolate the problem circuit.