Black Screen With Blinking Cursor.

This week when trying to boot my computer it gets stuck on a black
screen with a blinking cursor, if I hold down F12 to go to the hard boot
screen and select the hard drive it boots up okay and works fine.

It also if left on overnight will re-boot itself, and then gets stuck
on this screen, it did this to me last night after it finished running
the virus scanner. I checked the virus scanner history, once I managed
to hard boot it and it had finished running and hadn't found anything.

However when I managed to boot it this morning, it installed windows
update 3 of 3 so I believe it was windows updates that restarted it in
the night, but I have this set to only install the windows updates when
I tell it to not to do it automatically.

Anyone know of any recent updates that may be causing this to happen?

I also have an external hard drive plugged into the computer all the
time, could this be confusing it as to what disk to boot from? However
this has been connected to the computer since we bought it which was
over a year ago now, and haven't had any problems until now.

If anyone knows what I could do to check what it's trying to boot from
or anything else I can check I would be grateful.

Try running Malwarebytes antimalware.

In my experience an external hard drive connected to a USB can sometimes
confuse the BIOS and become the 1st boot drive. Enter the BIOS and select
your boot HDD in the Boot Order and then save the changes.