Black Screen with Meaningless Error Message!

HP Pro Book 455 with MINT 17.3 Rosa installed.

Occasionally my computer will shut down to a black screen with the following error message in the top LH corner of the screen:

Linux Mint 17.3 Rosa graham-HP-ProBook-455-G2 tty 1.
graham-HP-ProBook-455-G2 login: (flashing cursor)

This happened last when I attempted to enlarge a PDF file I had just down loaded. I try to log in with my password but it does not recognise it. The only remedy I have is to cut the power with the main switch and then re-boot. I am worried that this is damaging the system. How can I avoid this error or how can I return to the page I was on without shutting the computer down?

It looks like it’s dropped to a console (tty1) … does hitting Ctrl+Alt+F7 return you to your desktop ?

Thanks Mark, i shall try that next time it plays up.