This is mostly a problem with the GeoXT units. If the receiver will not connect to the satellites, it has probably been manually disconnected. To reconnect, go to setup > Options > Connect to GPS.
If you cannot see satellites when using the Hiper Pro, a number of problems could have happened. You may have lost the radio connection with the base, you may have improperly setup the base and rover, you may have lost Bluetooth connection between receiver and data collector, or you may have selected Total Station over GPS. You should either consult with the GPS technician (or if you've completed the training), double-check your setup and manuals.