I don't know what OS you have on your MacBook, or the year, so can only give general suggestion. Under 'About this Mac' then 'System report', the Dell should show up as a display.

choose either 30 or 60 hz, whatever resolution you can. See if the cable is HDMI 2.0 and HDCP 2.2 compliant.Under system preferences, then displays, go to scaled. On the Dell, you should be able to switch and prioritize the inputs. Yes, check the adapter (if you're using one) and/or the cable. I also presume you're not trying to get 4k out of the monitor-you just want a darn picture. I presume you are using an adapter to connect the Dell Monitor to your MacBook. However-Dell is known to not play nicely with Macs of any species. I used to use a Dell 27' monitor with my Mac Pro desktop, so I know it basically works. So is this monitor not compatible with MacBook Pro's from 2020 and if not, why did it work for a while but not now? I've made sure my macbook pro has the most up-to-date software/securities. The Dell monitor works with my husband's PC. That would rule out my HDMI cord and adapter being the issue. I've plugged my macbook pro HDMI cord into my Sony TV and it works as an extended display like the Dell monitor used to. The adapter I'm using is: USB C Hub HDMI Adapter for MacBook Pro / Air Swith and Other Type C Devices, USBC Digital AV Multiport (6 in 1 Dongle with Ports of typec USB 3.0/4K HDMI/SD/TF Card Reader) Type-c Multi Docking I've unplugged and plugged HDMI cords, power cord, adapter, etc. Then just yesterday I tried all the tricks that have worked in the past, but nothing made the monitor connect with the MacBook Pro. Then a few days later the macbook pro got where I had to restart it to get the monitor to work. For a few days I could unplug and plug things back in and it would start working. Then, all-of-a-sudden, I started getting the 'no HDMI signal from your device' message on the monitor. Dell monitor S2721H not connecting with MacBook Pro - says 'no HDMI signal from your device' My S2721H Dell monitor was connecting with my 2020 MacBook Pro for weeks with no issues.

Hdmi cord for mac book pro