If when attempting to access the TrueNAS Core management UI you just see...

"Connecting to NAS... Make sure the NAS system is powered on and connected to the network. "

The most likely issue is the middleware daemon has experienced a crash and may need to be restarted.

SSH and login to the TrueNAS core device as root.

From the CLI check if the middleware is running

service middlewared status

If the service is running, or hung, stop the service

service middlewared stop

Once the service has stopped, restart the middled service

service middlewared start

Note. This may take 30-60 seconds to start.