Laptop only displays a " _ " when I try to boot up

"Try these :

Change your RAM, or if you have 2 sticks of RAM, alternate plugging them out.
Plug out hard disk, all unnecessary peripherals. See if PC boots and POST.
Plug out battery too.
Put your ear to the notebook, any high-pitch or weird sounds?

Let us know."

Are you able to see the BIOS load? (Black screen usually with the brand of laptop logo) If yes you might be able to boot from disc and reinstall windows. Your going to lose everything if you don't have anything backup on a disc or drive. Ofcourse this would be one of your last options.

I had this happen when the computer is set to boot to USB before the internal hard drive, and you have an external usb drive (hard drive or flash drive). It tries to boot to that drive, and nothing happens. Either unplug any usb drives, or change your boot order in your BIOS.

"I had this exact problem on my computer a few days ago, it was bad RAM.

Same symptoms, no POST beep, just the flashing underscore.

Try taking out a stick at a time and see if you can get it to boot."