Fix boot hangs with /dev/sda1: clean ... files ... blocks in Ubuntu

If you have switched graphic card driver from Nvidia to Nouveau in Ubuntu, you might get this message after boot, where it just hangs:

/dev/sda1: clean, ***/**** files, *****/**** blocks

Solution:
Click ESC during boot
Select Ubuntu with Linux ... (recovery mode)
Select Resume normal boot
You should be able to log in, switch back to Nvidia end reboot