How can I fix my Potterton Netaheat 10-16?

Any ideas what's wrong with my boiler? It was working fine this morning but this evening there is nothing. Thermostat is set to high and thinks it's heating, but the boiler is not firing up. No pilot light, no nothing. I can't seem to get the cover off to look at the inside and follow any troubleshooting guides. Any help would be GREATLY appreciated as I'm freezing!