It could be the PMU playing up. Mine did this and the procedure for getting it up and running again is somewhat hit-and-miss.

From memory you need to unplug the machine from the mains, remove the main battery and then leave it for some time (the advice I remember is over-night but I seem to think that a couple of hours was enough). Then hold down the reset switch on the back for 30 seconds. Re-connect the power, hold down the reset switch again (for another seconds) and then try powering it up.

On my machine this worked for a while and then gave up the ghost (like you I removed the PRAM battery thinking that it'd help - I'm not certain whether it did or not)

If you do get anywhere with the fuse then I'd definitely be interested to know what you did