How can I fix my Greenstar CDi condensing combi boiler?

...after nearly 4 years of fault free operation from new, this morning I had no hot water & a "EA" message displayed. The burner indicator light was off & having turned the unit off then on, setting both the CH & Hot water setting to maximum, the system re-ignited & the CH system came on (display alternated between "-ll-" & a rising number?). The radiators heated up, however, when I turned a hot water tap on, the burner indicator light went out & the tap ran cold. I've repeated this a few times after turning the boiler off & the same thing keeps happening (the reset button doesn't rectify the problem once the burner indicator light goes out).
Can anyone help & advise what I should do to fix this?
Thanks!