Have you tried any of the guides (text, video). There are tons of guides.
It has been a while since I did this sort of thing, so I might be a bit foggy on the details. I would recommend you look at one of these guides.
However, if you prefer to do it on the forum, we can give it a shot.
Note that, be very very careful. I take no responsibility for any lost data etc.
There are a few questions:
What is your OS (Windows 7, Windows 8.x, Windows 10?). My main OS is Windows 7, so if you use 10, many things are similar, but some details might differ.
Do you have a DVD drive?
Do you have a recovery DVD?
Do you have a clean install copy of your OS?
The actual guide would be as follows.
Lets try the simplest stuff first.
1. Have you tried the inbuilt repair option?
[https
/www.youtube.com/watch?v=5Zkvlcnv-UU&ab_channel=Britec09 guide for Windows 10.] Do not reset! (at least not yet).
2. Do you have system
backup (not image) points, that you could go back to? I don't remember, but they might be on by default. I believe 10% of the main partition is for recovery by default.
3. (maybe 4. is easier?) If you have a recovery DVD for your OS, pop it in and boot up the PC. If you do not have it, you will need to download the recovery DVD from Microsoft with that OS version that you have. If you have an other PC with the same version of the OS, you can use that one to make a recovery Disk. If not, you can try to find one that is as close as you can to your OS (i.e. if you have Win 7 Sp1, try to find a Win7 Sp1). Make sure that its the correct bus. I.e. x64 (64-bit) or x86 (32-bit). I think windows 10 is only 64 bit. Than, burn it to a DVD if you have a DVD drive or make a bootable USB. I don't remember all the options there. Have a look and if you get stuck and can not find info on the web, come back here with a detailed list of options etc.
4. This might be easier than step 3.
YOU WILL NEED YOUR ACTIVATION KEY. Make sure you can get it. Dont do this if you can not get your activation key!. Also, make backups if you can at all. Cloning the HDD for example.
Pop in the windows installation DVD (or bootable USB with installation ISO). Than do a repair install. NOT! custom install. Basically, if at any point it ask you to format your drive, or warns you that your data will be lost, you are in the wrong section and you should stop what you are doing.
See for example
https/www.youtube.com/watch?v=RC_5eb9wTfk&ab_channel=Britec09 This is for windows 7. But the same idea work for windows 10.
If you do not have the installation ISO, you will need to get it. You can get it from Microsoft's own website.
I wrote a lot, and not very clearly either.
Basically, give us more info and try a few of the easier steps. Than, we can talk a bit more in detail.
Cheers.