A programme on Radio 4 stated that every time an area suffers thunderstorms, the local PC World replaces at least 30 modems on the following day. Modems are particularly susceptible to the electrical surges created by thunderstorms and they're easily 'killed'. (I've lost two modems that way myself. I know that you're meant to disconnect the phone line during a thunderstorm but I never seem to remember in time!). I'd be prepared to bet that your modem has suffered a similar fate. Fortunately, modems are fairly cheap, so it shouldn't cost very much to fix the problem.
Chris