There is a possibility that your hard drive connection was not making a real good connection. I had this one time but was able to solve like you did by removing the hard disk and reinstalling it. Other than that, it could be that your install of windows 7 somehow got infected with a boot virus, but if it is working properly now then I would not worry too much.
Just keep turning it off periodically and see if your symptons remain.
Have you had any problems since it did boot properly?