My Corsa 1.0 2002 won't start after ecu light on for a few days?

it was driving ok last week then this light with a spanner on it came on and wouldn't go off. I thought it was the colder weather. I have no heating in the car although the fan does blow out cold air quite normally. This morning I went to start it and zilch...the seatbelt light comes on on the dash and makes a continuous clicking noise..now there isn't even enough power to lock my doors with the key I had to lock them all manually. I have rarely had heat by the way, it comes and goes but went off a month ago and no heat since. The mechanic who put it through its service 3 months ago said he found some little bits of metal in the oil bit but he cleaned them out and said it should be ok for now..
so, do you think that light being on would have indicated possible battery problems coming up? The car has always started first time you see