I had that same problem on my Comcast cable connection nearly every day for about a month earlier this year. The only way to 'fix' it was to reset the equipment or just wait until it ended. Switching your PC from your router directly from the modem and cycling power on the modem to get it to work would have the effect of resetting the modem, so it sounds like you have the same issue.
After several weeks of phone calls and multiple phone jockeys denying anything was wrong, I finally got in touch with someone that was willing to tell me the truth, and it turned out that Comcast was shutting down and resetting all of their equipment in the area every night because they had over-sold their bandwidth so they were doing it to force the people running 'servers' and P2P software offline. They finally got the equipment upgraded about two months ago and everything has been working perfectly since then. It can't hurt to give your ISP a call and ask them what is going on, but I wouldn't be surprised if there was a similar situation in your area.