Monitor just stopped showing image with DP cable

[GameOver94]

My monitor stopped working with the DP connection after I connected another computer via HDMI. The computer that is usually connected via DP was turned off but still plugged into the power supply. I connected the monitor via HDMI to another PC for just a few minutes.

When I went to turn on the PC I usually use, there was no image, even though I had already disconnected the HDMI from the monitor.
I restarted the computer twice, but still nothing. I thought the GPU was dead. I tested with HDMI and immediately got an image.
After that, I tested another DP cable, but still no signal. I reconnected the HDMI, and it worked again.
I thought I might have burned the DP port on one side or the other at this point.
After disconnecting the monitor from the power supply, it was still the same. Only after unplugging both the monitor and the computer from the power supply did the DP cable finally work again!
It's the first time this has happened to me. The monitor is old but still works well. However, this situation really annoyed me today.
Could this be a sign of age, or does this happen more often with DP cables than I think?