The batteries do not power the thermostat, the batteries are to maintain its memory in the event of a power cut.
If the thermostat was working as it no longer displaying, its unlikely to be the wiring in the bracket behind, you can find the wiring instructions on page 3 (section 4) of the manual here.
I would start by checking the boiler has power, as thats where the power for the thermostat is coming from, if your confident in what your doing you can check your getting power from the boiler using a multi meter set to 240v and testing across the L and N terminals.
Based on what you have described I think its most likely to be a thermostat failure if you have power at the boiler, you can purchase a replacement thermostat from ebay here.
Do you still require assistance, I haven't heard from you in the last few days?
I havent heard from you in a few days, and wanted to understand if I can be of any further assistance?