Electrics

After I tested the in-tank submersible pump and filled the Truma water heater, I switched off the 12v supply and everything worked perfectly up to then. Next day, I discovered following switching on 12v that nothing worked. The Elektroblock in the wardrobe appears to be OK and charges in three stages.
I have replaced the leisure battery which I found was completely flat with a 115ah and this hasn't improved the situation.
I have since heard that at no time should a leisure battery should be allowed to go flat which could cause lead to damage. Can anyone offer a remedy and shed "light" on the problem.

Thanks for your interest. Is the relay you mention on the Ebl something I can re-set myself and where can I find it. Any help please.

Is it possible to open up the Eblektroblok and unstick the relay in question without causing other problems. Advise me please.

Thanks Tristan for the help.
I've located the 12V relay and it is faulty indeed. Is it possible it can be overidden by removing it and replacing it with a simple 12v toggle switch with a suitable load rating and appropriatlely located. Your help please.