Try to find the reason for what is going wrong, before attempting fixes you find on the net from other people that end up with a black screen. Because there are different things that can go wrong that can cause this, so there is no one solution.
First attempt to get a terminal with CTRL-ALT-F3 where you can login with your user. If CTRL-ALT-F3 does not work try it with other F keys
You might also get a terminal from the advanced option for Ubuntu in grub.
Once you have a terminal check that you do not have a full disk with the df command