Honeywell programmer program

Hi all,
Sorry the title should have said problem not program! My Honeywell ST6400C central heating and hot water programmer has just gone wrong. I took it off the base plate and cleaned out the dust then put it back and pressed reset.Now I can set the date and time and programme in the heating and water times but as soon as I go to "run programme" the display disappears and nothing fires up. Do you think this is a problem with the unit which will be fixed by getting a replacement? Or could it be a different electrical problem? It's going to be a bit hard to get a central heating engineer out at the moment so I'd like to fix it myself but I'm not going to muck about with electrics...
Many thanks
Sean

Could be a different problem but most likely the programmer. Replacement is an ST9400C, fits on the same backplate so no rewiring required

Have you checked that all the terminals are done up tightly on the back plate? Turn the power off first!