Desktop crashes on trisquel 5.0

Hello,

I'm new to trisquel and GNU/Linux OS as today I had problem with my desktop on DELL VOSTRO 220s as I loged in to my account. Computer was running slow and were looking true processes running and couldn't see anything that would cause computer to crash. When I try to open Firefox computer freezes for couple of minutes and doesn't open Firefox. When trying to open Thunderbird it also freezes but after couple minutes opens it and then if you want to open any other application or file the Thunderbird goes grey and everything disappears from desktop all applications and menu bar and doesn't respond to anything just can move mouse around.

I would appreciate if anybody could help out. Thank you in advance.