Hi Carl,
- What is the make and model of your computer?
- Have you made any changes to your computer?
Please disconnect all external devices and check if the computer boots to the desktop
Please use articles below and check if it helps.
Startup Repair
http://windows.microsoft.com/en-US/windows7/Startup-Repair-frequently-asked-questions
If startup repair fails, try a system restore from Winre
What are the system recovery options in Windows?http://windows.microsoft.com/en-us/windows/what-are-system-recovery-options#what-are-system-recovery-options=windows-7
If still there is any issue, follow the articles
Windows Vista or Windows 7 no longer starts, and the Startup Repair tool does not fix the problem
http://support.microsoft.com/kb/934540
Use the Bootrec.exe tool in the Windows Recovery Environment to troubleshoot and repair startup issues in Windows
http://support.microsoft.com/kb/927392
Hope it helps
Thank you