I see much the same at times (also Windows 7, but not sure if that is relevant).
Typically unplugging the Arduino and plugging back in and waiting for a few seconds sorts it out. However all the plug-unplugging will make any loose connections worse...
Strangely looking in System Manager shows the Port as present, just the IDE can't find it.... Also sometimes plugging it in does not show up in the UI, but System manager shows it (and you can hear the noise of a new USB connection). So Windows is recognizing it, but the IDE is not. (Strange).