I'm amateur repairman in our household but I can't seem to google what this problem is. My roommate used her laptop last night -- the battery ran low so she plugged it in, used it for a few minutes, shut the lid (so it hibernated) and then unplugged it, took it to another room and plugged it back in overnight.

When she went to use it this morning, it won't even attempt a boot. The lights on the right side blink -- power and either battery or cylinder -- and if I hold the power button down, all three of the locks at the top blink. One longer flash, and then quick flashes until I release the power button. Same deal if I use FN & power.

I'm terribly afraid it's the motherboard but it seems more like it would be a power supply issue. We've tried her battery in and out, attached and detached from the AC adapter, my AC adapter, etc. I also reseated the RAM in case that was the issue. Any other ideas? The laptop is several years old but aside from general slowness it was working just fine last night!

Try hooking it up to the AC, and remove the battery. Then, using a multimeter, see if the laptop's actually supplying power to the battery.