Ok well the tower could be down for maintenance. They did that here for a few hours and when the site came back up, it was using a new control channel. Check and see if you can hear a control channel for the VIPER site you are listening to. If not, it may have changed.
I know you said you can’t hear mitchel, but can you hear other agencies on VIPER like SHP? Are other VIPER sites working for you?