Emm.. you may remember the wrong password or someone hack your computer and change the password.
Anyway, you need to get access to you windows 7 now. You can make a Windows password reset disk to reset your password:
1. Login to a computer that can link to Internet. Download Windows Password recovery Tool 3.0
and install it on that PC. Note that: there is a .ISO file. Burn the .ISO file to a CD or a USB flash drive.
2. Get out the newly created CD/USB and insert it to the locked computer.
3. re-booted the locked computer (it's necessary for you to change your computer's Setup to make it boot from CD drive or USB drive) and then followed the process of instructions. Just a few steps, the old password was removed.
4. Setting new password.
won't do any harm to your data, only remove windows password
So, is it easy? Luck!
Related Tutorial: How to Recover Forgotten Windows 7 Password ? http://www.windows7password.net/forgot-windows-7-password/