Any of several things might be the answer. It could be that you haven’t set the location on your account properly, at www.sonos.com. It could be DNS issues, requiring a network refresh. It could be simple wifi interference getting in the way of your speakers connecting to the servers they’re being pointed to. Unfortunately, you’ve not given us enough information to be sure. I’d recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.