|What exactly do you see on the screen? Are you getting error messages?|
If you have Windows open, but it is completely frozen, you can manually shut it down by holding the power button of your computer for 5 or 6 seconds. Just be aware that any unsaved work will be lost.
If you can't even load Windows, how far does it get into Windows? Does Windows start to load, but then give you a BSOD?
This is when knowing the exact error message would be useful.
If so, you could try running Windows repair from your Windows 7 disc.
Have you messed around in BIOS lately?
Does your computer POST? Can you access BIOS? (usually done by pressing F2 or something)
Please provide the exact make and model of your computer.
If you answered no to all of these, it's likely due to a hardware failure, although why rebooting would cause this I don't know...it could be a coincidence.
Does the computer sound like it starts, but you see nothing on the screen? (fans spinning up etc.)
If not, it's likely a power supply issue.
User Info Page Biography for full specs.