Almost certainly the PSU as they are know to fail after all these years, you could try and find another power supply or just wire an existing Sonos speaker to your Router.

To be fair, my S1 system is over 10yrs old. Essentially the bridge is just a router and your normal router for the net only lasts around 5yrs anyway, so Iwas expecting it to pack in at some point. There's nothing wrong with the transformer,  I've checked that out, it's fine. I'm led to believe I can just bin the bridge and just wirelessly connect all my speakers to my home router/network and hey presto.....music! Can this be done on the S1 system? Thanks folks.

https://support.sonos.com/en-gb/article/connect-sonos-to-a-new-router-or-wi-fi-network
 

You will need to temporarily wire one Sonos device to make your life easier

Your (dead) Bridge will get you a 30% discount on a new Boost, if you need it.