Hi @xyyz
Quite a number of us have been having issues with the photon in this respect. (Although I can't guarantee the issues you're having are the same), I'd recommend following this thread:
Many of us have found it's reasonably common for the cloud connection to drop, but the WiFi remains intact.
From my experience, it's highly likely that if you were able to reset it, it would come right back online. (I know this doesn't help in your particular situation), but this is an area the Particle team are looking into I believe so stay tuned!