Originally Posted by
orinthia
There should be a hard-to-press button somewhere on the modem itself, usually needing a pen to press; hold it down for a few seconds to wipe the modem and let it reconfigure itself. Try this if your modem is a regular modem, ie. it isn't also a router, otherwise you'd be resetting the router part's network configs and such and you probably don't want to do that (backup those settings first and maybe try).
You could also plug in directly to the modem with your pc (if not a modem+router unit). See if stuff works. Try a free vpn, like a browser based one or otherwise. If directly connecting through the modem works but over router doesn't, shoot the router. If browsing those not-working sites over vpn works, isp has problems.