Edit mode refuses to activate All I did was select bunch of vertices and move them. Then I was supposed to deselect a few of them, but Blender has switched to object mode and I can't activate edit mode anymore. It's not on the list that has the modes and pressing Tab does nothing. Anyone know how to avoid this from happening?
Like stated in the above comments you probably have more than one object selected, or you might have selected an object that is not editable (eg a light, camera, )Try pressing A a few times and then reselecting your mesh, If you believe it to be a  bug, Try closing blender, starting a new file and importing everything into your new file.