The router has a limit on how many simultaneous telnet connections it can support (that number may be two). It appears that something has caused the router to think that it already has the maximum number of connections, so it rejects any more attempts to log in. If when you last closed down Windows, there were two connections already there, then it could be that these weren't disconnected on shutdown.
If you've been using Routerstats, you should always press its stop button before closing down, because it stays connected via telnet all the time it's running. If you've been logging in manually, you should always logout when you've finished, for the same reason. In the case of DSLstats, it's possible that you shut down the PC while it was in the middle of sampling, which could possibly also result in a stale login remaining.
All this is speculation, but I can't think of anything else. If I'm right, then I'm afraid that rebooting the router again is the only cure.