Try using a USB boot stick to boot into Linux and just save whatever you need from the machine and then reformat it. Since this way the machine won't automatically connect to Wi-Fi and potentially cause more issues.
That's the quickest, simplest way without needing to try to diagnose and dig into the system to see what is affected and trying to fix it.
Also what is the computer doing when it boots up? There's not really enough information being given to be able to provide any other advice.