Have you made any changes to the computer prior to the issue occurred?
You may disconnect all the external devices and check if you are able to boot in normal mode or safe mode.
Start your computer in safe modehttp://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode
If you cannot perform startup repair then you may try using the Last known good configuration. It is a Windows startup option that uses the most recent system settings that worked correctly. Every time you turn your computer off and Windows shuts down successfully, important system settings are saved in the registry. So it starts Windows with the last registry and driver configuration that worked successfully.
You may refer to the below links for further assistance on the issue -http://windows.microsoft.com/en-US/windows7/Using-Last-Known-Good-Configurationhttp://windows.microsoft.com/en-US/windows7/Advanced-startup-options-including-safe-mode
If you are not able to boot to the desktop, then try the steps from the following links:
Windows Vista or Windows 7 no longer starts, and the Startup Repair tool does not fix the problemhttp://support.microsoft.com/kb/934540
How to use the Bootrec.exe tool in the Windows Recovery Environment to troubleshoot and repair startup issues in Windowshttp://support.microsoft.com/kb/927392