A couple of suggestions.

First (which may be the case, it’s hard to tell), make sure all Sonos devices are powered off when you reboot the router. It ensures they’ll get new IP addresses after the router is rebooted, and forces a reload of the Sonos OS. And, as a side benefit, reboots the device the Sub is ‘bonded’ too, which may currently be in a state where it isn’t sending information to the Sub. It does take two to tango, as it were. 
 

Second, in case there’s a hardware issue with the Sub (both of my Gen 2 subs are working fine), I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

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.

Power down all Sonos, reboot the router (to clear old data) then power up your Sonos. If the Sub is still silent, play something with good low frequency content and submit a diagnostic. Call (by far the best option) Sonos and have the diagnostic looked at, they can see things we users can't. 

Thanks so much, this worked! 

If it keeps happening try setting static/reserved IP addresses using your router's DHCP Settings page.

That usually fixes it, the router page usually has instructions and it only takes a couple minutes.