How can i fix my baxi 105e combi boiler?

when i switch boiler on, central heating works fine, but as soon as i use hot water from tap, central heating light goes off and powers down completely. Rads go cold and central heating will not fire up at all. if i switch boiler off completely, leave for a period of time, it resets and back to square one. So it only lets me use one or the other basically. Any ideas? Please help!!??