If you are unable to boot into Windows, there are several troubleshooting steps you can take in order to determine the root cause of the problem and possible solutions.
The first step is to identify whether the computer itself is having a hardware issue or a software issue. If the system will not power on or if it fails to POST (power on self-test) then it is likely that the issue is with the hardware components and not the software. In this case, you should inspect the cables, RAM, power supply and other components to ensure that everything is connected correctly, and that all of the parts are in working order. If any hardware components are found to be defective or damaged, then those should be replaced before attempting to boot the system again.
On the other hand, if the system successfully passes the POST phase and powers on, but you cannot boot into Windows, then the issue is likely a software problem. In this case, you should try to boot the computer in Safe Mode (pressing F8 as soon as the BIOS loads during startup). This will allow you to access the Windows Operating System with minimal drivers and services running, which may allow you to identify the cause of the problem more easily. If Safe Mode boots successfully, then you may be able to disable any newly installed software or services to troubleshoot what is causing the problem.
If the system still will not boot into Windows even in Safe Mode, then you may need to perform a repair installation of the operating system. You can do this by booting from the Windows installation disk and using the Repair option. This will reinstall the operating system without affecting your personal data and files; however, you will need to reinstall any third party applications.
If the system still will not boot after a repair installation, then you may need to perform an in-place upgrade of the OS. This process will allow you to keep all of your personal data and applications while upgrading the operating system itself. Unfortunately, you will need another operating system installation disk in order to perform the upgrade.
If the system still won’t boot after the in-place upgrade, then it is possible that there is a hardware issue causing the problem. You should inspect all of your computer’s components once again to ensure that they are functioning properly. If any of the components are faulty or damaged, then you should replace them before attempting to boot the system again.
Finally, if all else fails and nothing seems to work, then you may need to perform a clean install of the operating system. This process will completely erase the hard drive and reinstall the operating system from scratch. You will need to back-up all of your files and applications beforehand, and then reinstall them after the clean install is complete.
In conclusion, if you are unable to boot into Windows, there are a few different things you can do in order to troubleshoot the problem. You should first try to identify whether the issue is a hardware or software problem, and then attempt to boot into Safe Mode. If this does not work, then you may need to perform a repair installation or an in-place upgrade of the operating system. If none of these steps work, then you may have to resort to performing a clean install of the OS.