Could be lots of reasons. That is unfortunately a very broad question. Here's what I would do if it happened to me. I am comfortable performing serious repairs on my laptop and you may not have the experience/tools to do that. If you are not comfortable, take the machine to a repair shop.
- Create a bootable USB repair drive. There are many our there (search on bootable usb windows repair tools). You will likely need to change the boot order to get the machine to boot from the USB. How you do that depends on the laptop. On mine, I can press F12 during startup and it will ask me what device I want to boot from. Yours will likely be different.
- If you can boot up the repair SW, you likely will have tools available now to...