From qubes manager: What is the template sys-net depends on?
A reminder: no software is provided under a qube. Templates provides software to qubes. qubes are just “user settings” disks and configurations applied on top of templates. So if you say that NetworkManager disappeared recently, something was either changed from qubes (qvm-prefs, settings from qubes manager, services deactivated etc).
The real question is what changed before/after NetworkManager disappeared from sys-net? That might be a hard one to answer, but something had to change.
Is your wifi/ethernet device showing under sys-net’s settings’s devices tab?
You could try to switch sys-net’s template to another one you have currently available and do emergency suspend from gui-domains (Qubes widget near clock wher eyou see all running qubes) and then ill sys-net. Open qubes manager, select sys-net, change its current template to another one. And then start sys-net again and see if widget shows.
Have you followed How to create new sys-net? ?
You could tell qubes manager to not start old sys-net by default under settings. Reboot. Create a new sys-net2 following How to create new sys-net? (selecting provides network at qubes creation), go under sys-net2 settings, make sure all networking devices are associated with sys-net2 and none are selected under old sys-net.
You will have to tell us a bit more of what you did before sys-net stopped showing NetworkManager so that willing people can help you troubleshoot and resolve your issue. Reading this thread, even if I want to help you, there is not much I can extract from previous posts. What happened?