Restoring it back to last know good configuration.
When you press the power button, start tapping the F8 key. You will get advance boot options, use the ARROW keys to select Last Known Good Configuration (your most recent settings that worked), and then press ENTER.
This is what the screen should look like
Windows Advanced Options Menu
Please select an option:
Safe Mode
Safe Mode with Networking
Safe Mode with Command Prompt
Enable Boot Logging
Enable VGA mode
Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Debugging Mode
Start Windows Normally
Reboot
Return to OS Choices Menu
Use the up and down arrow keys to move the highlight to your choice.
If that does not work and you want to reset it back to factory settings the info below will help.
Since you did not list the make and mode, this is a generic answer. Some or all of it might help you.
Make sure you back up all your files before doing this.
If your resetting a lappy, make sure you have the AC adapter plugged up. You do not want this process interrupted. This should take one or two hours. It is best to check on it to make sure everything is going fine.
If you can boot into Windows, click on start and then open your control panel. Then click System and Security. Under Action Center click Restore your computer to an earlier date. Then click on Advance recovery and select return computer to factory conditions. Answer any question and the system should reboot. Answer a few more questions and then it should go on autopilot and reset your system back to factory conditions.
If you can not boot into Windows then try Advanced boot options. When you press the power button, start tapping the F8 key. You will see the advance boot menu. Select repair your computer. (This option is available only if the tools are installed on your computer's hard disk.) Select your language and then enter your user name and password. Then click on (brand name) factory Image Restore and next. Answer the question and it should start the recovery back to factory settings.
Recovery from the recovery partition
When you press the power button start tapping the F key for your puter and the BIOS menu will open. Select recovery from the menu, answer the question and the recovery should begin. Depending on your puter this will take one to two hours.
Manufacturers BIOS menu F key
HP/Compaq ESC
ASUS Del
Sony F2
Samsung F4
Gateway, eMachines F10
Acer, Dell, Fijitsu, Gigabyte, Toshiba F12
Your BIOS menu should look something like this. Yours might be different. I am posting this to give you an idea of what it looks like.
ESC = BIOS menu
F1 = System Info
F2 = System Diagnostic
F9 = Boot Device options
F10 = BIOS Setup
F11 = System recovery