I accidentally paused the Linux display process

1. Switch to a new TTY. See How to switch between tty and xorg session? for tips on how to switch TTYs.
2. Determine the PID of the cinnamon process: ps -e | grep cinnamon
3. Send this process the SIGCONT signal with kill -SIGCONT [pid]

Short, concrete version of CameronNemo's answer:

ctrlaltF1 ^{_{# Switch to TTY1}}
user name↲ _{# Log in on this terminal}
password↲
killall -SIGCONT cinnamon↲ ^{_{# Send the continue signal to the process}}
ctrld ^{_{# Log out on TTY1}}
ctrlaltF7 ^{_{# Switch back to the graphical environment}}

F1 and F7 here assume that you have X running on TTY7, as is standard on Ubuntu and, I assume, also on Mint.