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.