Potterton Profile 100E problem - Please Help !!?

In the last few days the boiler has stopped working properly. I switch the boiler on and the boiler fires up successfully, however within 30 seconds the boiler stops operating. The consquence is we've now got no hot water, I've resorted to having the emersion on to maintain hot water for the family. I've found the reset button located underneath, this seems to be in a fixed position, i.e. you cannot push it upwards. Any help would be much appreciated.