Flat battery - no start!

My battery has discharged itself over last 4 days. I've coupled up my Halfords booster and the lights work ok, but there's no dashboard display and I can't turn the ignition key! Seems like it's shut down 90% of electrics. Is there anything I can do, or am I looking at a very expensive visit from an auto-electrician who's probably never seen a supercustom before?

you could have possibly blown a fuse to the dash using the booster,can you try another battery?

If you can't turn the key, if its an auto transmission, it might not be in P so press the red button on the auto change and put itinto Park.

Thanks for replies, will try everything - and let you know results!

Just spent £100 on new battery & everything now works fine - but the terminals had been butchered and the pos was upside-down ( wide taper up ) The old battery was down to 5.8 volts! Will keep a close eye on it for a week or two.......... Er..... Managed to turn ignition key by wiggleing wheel against steering lock

Update - found a possible cause of flat battery..... Was checking oil. etc, and noticed that the door light ( driver's side ) was coming on with the door shut! Have wiggled the contact and it seems to work ok now, but I'll investigate further when time allows.

Update: Put old battery on charge for a week - voltage crept up to 7.8, so it's now switched off & will get a decent funeral in due course....