Laptop won’t boot

I am trying to get a ThinkPad T21 2647-DAU laptop to boot. The A/C adapter is plugged in and it shows the battery is charging. When the power button is pushed, the HD light flashes just for a second, then nothing, not even the bios screen. I’ve removed the battery, HD, and memory all at the same time, then put them back and got the same results. I’ve heard of some special key sequence you need to press to get it to boot. Any suggestions would be greatly appreciated.