What exactly happens when you press the power button? Any error message? Please try the following:
(A) The moment you press the power button, start tapping the F8 key to get to the advance boot menu. Here try safe mode and last know good configuration separately one at each time and see if it boots.
(B) If you get to the desktop, run an updated antivirus scan on your system.
(C) If it does not boot, boot your computer from the recovery DVD and choose startup repair as per steps given in http://www.askiyogi.com/windows/16918.html given below:
Step 1: Insert the Windows 7 DVD into the computer. Restart the computer.
Step 2: Press any key when prompted to boot from DVD.
Step 3: Select your language, keyboard and currency from the drop-down menus.
Here’s to Good Karma