The MBR is the master boot record, and when you fire up your PC the first thing the PC does is look for where the operating system is located. It finds this by looking at the MBR. If there are any problems with the MBR, the operating system cannot be loaded and the PC will grind to a standstill.
You may get a message indicating that there has been a boot failure because the operating system cannot be located, or you may be given no indication at all of what has gone wrong and face only a blank screen. The error may be MBR error 1, MBR error 2 or MBR error 3 (or MBR 3 error). All of these point to corruption or failure within the master boot record. Restarting your system from a restore point should be the next step.
The cause of such a problem basically boils down to two things. The less likely one is that your PC is trying to load the operating system for an external device or drive, such as an external hard disk, and finding that it can't reboot because there is nothing there. Try disconnecting the external drives to see if that is the problem.
More likely, the master boot record has become corrupted or gone missing for some reason. If this is the case, the ideal solution would be to restore it along with any other missing files from your system backup device. However, many people have mislaid this or have never made a backup, so it will not be an option for them.
To fix MBR Error 1, MBR Error 2 or MBR Error 3, you will need to enter the Recovery Console. Type "fixmbr" and then press Enter. If the master boot record has been corrupted, this command will fix it. You will then need to remove the system CD, type "exit" and then restart the machine.
Wondershare LiveBoot Bootable CD/USB has a cluster of state-of-the-art tools to deal with this and most other problems on your PC, and should be kept handy in case of emergencies.