Hi Simon84,
1) Do you recall making any changes to the Windows prior to the system crash?
2) Do you get any error message?
Method 1: Boot the computer in safe mode and then check. To do this, follow the steps below:
Access the Advanced boot options menu by turning on your computer and pressing the F8 key continuously before Windows starts. On the advanced boot options menu select Safe Mode
Advanced startup options (including safe mode)http://windows.microsoft.com/en-US/windows7/Advanced-startup-options-including-safe-mode
Let me know if you can boot the computer in this mode, else try Method 2
Method 2: Follow the steps mentioned in the article below and then launch startup repair
Windows Vista or Windows 7 no longer starts and the Startup Repair tool does not fix the problem
http://support.microsoft.com/kb/934540
To launch the start repair, you may follow the steps mentioned in the link below; See the section “To open the System Recovery Options menu on your computer”
What are the system recovery options in Windows 7?**http://windows.microsoft.com/en-US/windows7/What-are-the-system-recovery-options-in-Windows-7**
Method 3: You may refer the steps mentioned in the link below to perform system restore.
Resolving stop (blue screen) errors in Windows 7http://windows.microsoft.com/en-US/windows7/Resolving-stop-blue-screen-errors-in-Windows-7