When you get to the sign in window press and hold the shift key then click power button and select restart. You should now see the Advanced Start-up Options menu screen. From the troubleshoot tile try advanced options then system restore. If you get back to normal then the first thing to do is create a restore to factory medium. Used to be a set of DVDs but now also allows a backup to a flash drive. I would do both if allowed.
Samsung Galaxy S8 review