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
otherwiseIf you can get the windows 10 desktop
right click on the start button and select SETTINGSthen accounts and recoverythat 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 sideif you hold the shift key on the keyboard down while selecting the restart optionit 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 pagemake 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