What's happened is you have windows registry corruption. Which can happen when overclocking, I know, because it happened to me.
You didn't do anything wrong really. Part of overclocking is instability, which crashes your system. The crashes, overtime can corrupt your windows installation.
You can try to run a chkdsk to see if that can get you back into windows. Try Safe Mode also. If you do make it back into windows, I recommend a backup of data you want, and create a restore point in windows.