I noticed the uber thread on this was locked so I couldn't post how I solved this.
I discovered a problem in the firewall settings that had been changed by source unknown (Windows Update?) that was preventing communication between my Sonos One and the PC.
Rather than mess about with the fire wall settings, the simplest solution is to go to the Sonos support page (https://www.sonos.com/en-gb/support) there is an en-us version if you swap it in for en-gb in the URL.
Download and install the Windows Controller:The setup for this app will fix the firewall settings and you should be able to connect again.