If you can Start your computer but can't get Windows to open, you are probably having a hard drive problem. There is about 1/3 of a chance that it is a software problem, however. So you try to get into Safe Mode and can't. Now what?
On some versions of Windows, including Windows 8, you can put ANY legal copy of Windows into your DVD drive and restart your computer. Depending on your operating system, the DVD will allow you to go into Safe Mode, and various repair modes, before it asks for your Windows Key. No, it is not illegal, because you are only using it to access your own legal copy of Windows.
On Windows 10, it is ridiculously easy to make a recovery disk. Get an unformatted CD or DVD and type "Windows 10 recovery disk" into any search engine. And if you're already fouled up, you can make it on another Win 10 computer, or even borrow one, because they're all identical.