Disk light the floppy or HD light?
Had to replace the original IDE hard drive on an A4000 a couple of years ago with similar symptoms (HD light). It would boot up only sometimes, then less often, finally never. Now it's quite reliable.
Can you boot from a floppy (OS disk)? Might use a cleaning disk on your floppy drive 1st if you haven't used it much.. Might even need to unplug the offending hard drive.
Boot from another drive in the early startup menu (hold both mouse buttons down)?
if that's not it, unplug all cards, try again..
hardware debug time.
