Your computer is experiencing a no boot problem which means that the operating system is corrupted. You can do a start up repair to fix the computer but if it won't fix it then that's the time that you need to reinstall the whole operating system.
It could have been very helpful if you provided us with your operating system so that we can provide you with the exact troubleshooting. However, I just presume that you are using windows VISTA or Windows 7. However, this troubleshooting also works with windows XP.
Before you start the start up repair. You need the Operating System Installation disc from Microsoft.
1. Insert the disc in the dvd/cd drive.
2. Restart the computer.
3. Once you see the "press any key to boot to cd" message. Press any key.
4. It will bring you to a page where it will ask you to INSTALL or to REPAIR. Just click on repair.
5. It might ask you to load the drivers. Just click ok or next.
6. Follow the on-screen instructions.
However, if start up repair won't fix the problem, Clean Reinstallation is the next step. Clean reinstallation will delete everything in your computer including the files, pictures, etc. that you have saved. But you cannot really access them anymore since the computer won't boot up.
I'm am providing you with the links to the tutorial on how to reinstall the operating system.
1. WINDOWS XP REINSTALLATION TUTORIAL
2. Windows 7 Reinstallation Tutorial
3. Windows Vista Installation Guide
I hope this helps. Thank you for using fixya.