this is after the windows 10 update ?
you can try these instructions. they give a bunch of ways to try to get to their repair functions where you can either try the repair or going back to your old windows. sometimes it works. but those are about your only chances if it wont even get to windows. sometimes its so screwed up your only option is to use the computer recovery options to wipe everything out and go back to factory specs.
If this is the first time it has started after the upgrade and you have not done so try just restarting the computer. sometimes that straightens it out
If you can get the windows 10 desktop
right click on the start button
and select SETTINGS
then accounts and recovery
that should give you repair options you can try
if you cant get anywhere on the desktop but you can get to the sign in screen
at the sign in screen before you log in
there should be a button to restart the computer on the right hand side
if you hold the shift key on the keyboard down while selecting the restart option
it should bring you to a page with some repair options that you can try.
if you can not get the computer to do anything useful at all than you need a boot disk ( actually a flash drive)
you can create the flash drive using the program that you download from this page
make sure that you select the option to create a flash drive for use on other machines
you can put that flash drive in the computer and restart it and it should reboot from the flash drive that will give you some repair options you can try including going back to your original operating system or re installing the windows 10 totally
this post shows all of the options that you can use from the windows boot drive including how to make sure that the computer will actually boot from the flash drive