Monitor working intermittently

Hello,

I have an external monitor - a Dell P2721Q. I have had it for over a year. It's plugged into the wall and then I use a USB-C cable to connect it to my 2017 MacBook Pro. I use the USB-C cord both for connecting to the monitor and (through the monitor) powering my Mac. A couple days ago, when I booted up my computer, it was getting the power coming through the USB-C cable, but the monitor says "No USB-C signal from your device."

I tried a bunch of things (listed below), and the monitor started working again spontaneously tonight, but then it stopped again. I also tried to backup my Mac using Time Machine, but it wouldn't register that I had plugged in my external hard drive.

Things I have tried:

1. Unplugging and replugging every possible cable (with lots of waiting 5 minutes, holding down buttons for 20 seconds, etc.) in every possible port
2. Switching to a USB-B <> USB-C cable
3. Update to latest Mac OS
4. Reset SMC
5. Reset PRAM/NVRAM
6. Look at the settings in the Display > Detect Displays (the monitor is sometimes showing up there and sometimes not, but even if it is detected, it's not working)
7. Everything on this page: If your external display isn't detected or appears blurry when connected to your Mac - Apple Support