Is there any power to the operating unit (e.g. lights or display digits) Check if a fuse has blown, refer to the technical manual for your heater that the correct amperage fuse and type has been used. If you replace the fuse, ensure the correct amperage is fitted. Should the fuse still "blow" then you must contact you local Eberspacher distributor for further assistance.
Do not under any circumstances increase the amperage of the fuse in an attempt to cure the problem. The correctly rated fuse is there to protect the heater and the wiring in the event of an electrical short circuit.
Remove the ignition glow plug and check coils for continuity or excessive build up of carbon across the coils.