I've just experienced the same issue, solid power light, nothing else.
It happened after an update, the computer was working perfectly fine before.
The first time I held the power button for 10 seconds ( quick power light flashes ) then rebooted ok.
Started software update again, same story wouldn't reboot.
This time holding the power button for 10 sec when powering on, didn't fix it.
I tried holding the eject button and the power button, even if nothing was inserted and it worked!
Heard the chimes and it started flashing some internal device ( one of the updates ) and then it restarted ok.