P2415Q, HDMI input not detected

I have a P2415Q that I've been using a few years at the office via a Thunderbolt dock and DisplayPort or mDP connectors. Now I'm working from home and I don't have the dock so I have a HDMI connection straight from the monitor to my Dell laptop.

• The HDMI cable is fine. I've tested it with the same laptop connected to a different monitor/tv.
• The laptop is fine. I've tested it with a different HDMI cable to a different monitor/tv.

The HDMI input worked fine for a few hours. (!!)

Then at one point it just ... lost connection. "Entering PowerSaver Mode"

I unplugged all cables and left it alone for 30 seconds.

Plugged back in the power. Turned it on.  (got the message that no HDMI detected and will power save in 5 minutes)

Plugged in the HDMI to the monitor. (No change in the message)

Plugged in the HDMI to the laptop. The message changes to "entering power save mode" (5 minutes had not passed) and the screen goes black.

Help?

This Dell laptop looks like it only has HDMI and USBC/Thunderbolt(?) ports as available display ports.