This First part only works when a valid Recovery Partition is enabled. If it displays an error message then move onto the Second Part:- OR if you have data you wish to try and save using the XCOPY command. Then create your own Repair/Recovery/Rebuild/Re-install media or buy a set from HP.
Turn off the computer. Disconnect all connected devices and cables, Media Drives, USB drives, printers, & faxes. Remove media from internal drives, and remove any recently added internal hardware. Do not disconnect the monitor, keyboard, mouse, or power cord.
Turn on the computer and repeatedly press the F11 key, about once every second, until the Choose an option screen is displayed, and then continue.
On the Choose an Option screen, click Troubleshoot, and work your way through the screens.