My TC (latest revision) suffered a similar problem once, although weirdly I could happily surf the Internet but couldn't locate or connect to the TC HDD. When I took my MBP upstairs nearer to the TC I could access the HDD (so maybe a signal quality issue). Either way, I reset the channel selection by selecting manual, restarting and selecting automatic again (and a further restart). That seemed to solve it and I've not had any issues since, so maybe worth a try?