Hi,
Please do try these troubleshooting steps that may help.
- Start with a hard reboot, remove the battery and unplug its AC adapter then press and hold the power button for 20 seconds then try booting it up again.
- If it stays on the black screen again, press CTRL+ATL+DEL and select Task Manager.
- Once opened, click on File then select "New Task", once Run is opened type in explorer.exe and click OK.
- If your desktop screen will come back, press Windows + R then type in "msconfig" and click OK.
- Once System Configuration is open click on Startup tab then disable all the startup items and reboot.
- If it doesn't work try loading your laptop in Safe Mode, while your laptop is booting up repeatedly press on F8 until it loads Advanced Boot Options and select Safe Mode.
- In Safe Mode open System Restore and restore it back to an earlier date or time before the problem even happened.
- If all these will not work your last option would be to run System Recovery to reinstall Windows 7.
- Here's how to run System Recovery:
http://support.hp.com/us-en/document/c01867418