Desktop size suddenly shrunk.

[Mute]

I left my room for ~15 minutes, and when I came back, the desktop window's size collapsed to fill only a small window on the screen, leaving a large black area around the desktop, and a message was displayed saying that because my computer's memory was low it wanted me to close firefox.
I understand my system's memory might be low, but how do I revert the size of my desktop back to normal? Changing the resolution does not solve this issue.

[Simcaz]

Did you try restarting your computer?

[Mute]

Did you try restarting your computer?

I'll try once I'm done shifting some stuff (that's admittedly large) to an external hard drive.