I just installed Ubuntu 18 on my desktop PC alongside Windows 10 and have been using Ubuntu without any problems for a couple of days. Today, when I booted up my computer, I am no longer able to open the terminal at all. I've tried restarting it multiple times, but it still won't work. If I click the terminal icon on my task bar, nothing happens. If I try opening the terminal with `CTRL`+`ALT`+`t`, it still doesn't work. 

How can I resolve this?