Pull the battery, leave the power disconnected, remove the RAM and hard drive.
Reconnect the hard drive, then the RAM, and then plug in just the power cord after 15 minutes of them being out. Turn the system back on and see if it posts normally. If not, pull the RAM out and try it one at a time. If still nothing, it's possible the system pooped out on you and finally kicked the bucket entirely.
Alternatively, try some other RAM, but if you still can't get any video on both an external monitor and the onboard LCD display, then you'll have to cut your losses.