What happens when:
1) File > Go Online ?
2) Exit & Restart FTM
3) Exit programs & reboot machine
You didn't say if you were online with your browser or wifi or whatever, on that device. Was your browser on-line and FTM off? Try 1) above.
It may be a wifi problem. Sometimes I lose a wifi connection for no apparent reason at all. It often reconnects if I wait and try later, or force the WIFI to recognize my device again.