I had similar issues while using a 4-port hub UNPOWERED connected to a 15 foot USB cable both with my keyboard and mouse. Sometimes one, sometimes both. Sometimes replug would bring it back, other times it wouldn't (until a reboot).

My whole setup has changed since then (new mobo, no longer using a hub nor 15 foot USB extension cable). So I have no idea if it was an issue with the mobo, the peripheral or just the flaky OS (Windows of course )