Before you replied I simply turned both the laptop and the monitor on and off again. That worked. Always forget that 90% of a tech support person's job is telling people to turn it off and on again. A little perturbed that Dell Inspiron support wanted $99 to fix it. If I had paid $99 for someone to tell me to turn it off and on again, I would have been pretty upset!
This morning for some reason my Dell S2721DS monitor started going dim every 30 seconds if my mouse is not moving. The screen on my  Dell Inspiron 7706 2-in-1 laptop does not go dim every 30 seconds. I haven't changed any settings. Windows was updated a couple days ago, but that's pretty par for course. Windows seems to update every 2-3 days. Have checked display settings for the monitor and it is set to never go to sleep when it is plugged in, and to go to sleep in 2 hours if it is not plugged in.
S2721DS, going to dim every 30 seconds