how can i sort my halstead ace boiler?

Wish I'd not touched it... It was running OK but I noticed pressure on gauge was only 0.5 I was told keep it around 1.5. So I tried to fill it up a bit, the only problem the little tap had broken off were I turned it on so I used pliers to open it and the water started flowing n putting pressure bar up, I couldn't turn it off quick enough n filled it up too much, I managed to turn tap off so I could no longer hear water running in, the heating was still working but water was running out the overflow outside, I left it for a while then noticed the pressure started falling but it went to 0 so I put a bit more in up to 1.5 it kept slowly dropping until the boiler cut off, now I can't get it on it lights for a few seconds then goes off the pressure won't stay up....HELP