The first thing to do is don't panic.
The second thing is grab a cup of your fave working poison and strip the 4k of every add-on possible. Boot up and see if you get anything. If it boots ok add in one of the add-ons then try again.
You'll know this by now anyway, but it's the best way to see what's wrong. And if you're in a panic, you don't think of this :-)
When my A1200 accel's FPU is playing funny buggers I get the same symptoms as you. It could be something simple like this (well, "simple" it isn't, but better than a whole mobo going).
