Hi! could be a cia chip or the Paula chip I tend to think it may be a cia chip as you cant get to the early boot screen sounds as if your mouse has stopped working ( cia related). I lost my mouse once on my 1000 and replaced the cia chip and it worked perfectly now the bad news the cia chips are not socketed on a 1200 and would probably have to be replaced by a techo unless your pretty good at soldering? try swapping the Paula first and see how you go you may be lucky.
cheers
