Back your stuff up before you do any fiddling! remember to get any licence info for software on there you might need to reinstall and get hold of a win10 iso image to stick on usb or dvdrw - loads of guides available on how to reinstall if it comes to it, you should be ok with the windows licence as it's part of the bios and MS account info nowadays.

To see what's going on at startup, right click on the 'not the start button' (what do we call it now?), choose computer management>event viewer> windows logs>system, find the time you last restarted (easiest to force a reboot and then look here), then it's a case of trawling through back to current time, looking for errors and warnings and googling them if they aren't clear. It might turn out to be a duff driver or corrupt files or all sorts of stuff other than a dying drive...

While in computer management, you should see a storage>disk management section which will give you quick overview of your drives and will show the status as reported by SMART monitoring, hopefully it says 'healthy' and not something like 'pred fail'.

Also, open file manager, find the drive, right click and choose properties then on the tools tab, 'check' will run a file system check - if there are problems it might trigger some further steps.]

I guess that's enough to get you started and find anything obvious, there's also some diagnostic stuff in the bios but I can't say I've ever used it, hopefully someone else can give you some pointers there.