My laptop won't go past post. I have a black screen with on underscore on the top left. I've done everything I've read about rebooting. F2 I did the default saved and exited. F12 I did a Diagnostics and reboot hard drive. Everything looks good. I even took it all apart put it back together and still nothing. I read to do a factory reset F8 I hit on start up and nothing. I need help