This usually happens to me as well. However, in order to fix it, all I would do is go into my control panel, then sounds and audio devices.
From there, click on the Voice tab, and there should be a button to "Test Hardware." Upon completing the test, it usually will recalibrate things and your mic should be functional in TS. Before running the "test hardware" feature, I would suggest you completely close out of TS or Vent prior to using it.