which antivirus have you got loaded and is it free or paid version.
is this what you have below? and did you click on boot time scan to run on next boot up.
if yes, then this is a known issue with boot time scan and can be reverted back to the original boot time scan in hidden geek area.
if after every time you reboot this shows up then i cant remember how i actually got out of it but turning pc off, removing power cable for 15 minutes then trying again may, i say MAY clear the memory.
once you can log in again hunt down the he=idden geek area here
https://support.avast.com/en-en/article/Use-Antivirus-Geek-settings/#pcdont tick or untick anything else except in my second picture. once unticked then reboot and when you run boot time scan next time it will use the older version with a black screen.