20.04: screen frozen, alt+ctrl+fn does not invoke tty

I had opened 6-7 application(the memory is 16GB) on ubuntu 20.04, just as I opened a new tab in firefox, the whole screen was stuck-up. The screen was still showing the GUI of firefox, but when I moved mouse or pushed keyboard, the screen did not change anything(and the keyboard would light up when I push a key, but the keyboard light is set to off in my seesion). I tried Alt+F2, Alt+Ctrl+F1-7 and Alt+Ctrl+Fn+F1-7, none of these brought any change to the screen.

I have also tried SysRq key, but I am not sure which key is SysRq on my machine(Dell vostro 7500).

Now I have powered off my machine and restarted it, but I do not want to do this every time when I encounter this problem(this problem has happened several times).