This problem often happens when data on the hard drive is corrupted or not found. First, check your BIOS settings to insure the hard drive is active and that the proper partition is selected. Secondly, make sure you don't have any external USB devices attached to your computer. If the USB flash drive is corrupted or has crashed, it may cause the boot process to slow or not start at all.
To resolve this issue, you should perform the following steps:
Eject the flash drive from the computer.
Insert the Windows 7 installation DVD in the disc drive.
Start the computer.
Press any key when there is a prompt to do so.
Select language, time, currency, keyboard, input method, etc.
Click the Repair your computer option.
Click the operating system that is to be repaired.
Click Command Prompt in the System Recovery Options dialog box.
Enter Bootrec.exe/fixmbr and press Enter. This command would fix the MBR issue.