At the bottom right hand corner of the screen to the left of the xyz coordinates you will see the word  SNAP. If this is grayed out then snaps will be inactive. Left clicking it toggles snaps on and off. Right clicking on it will bring up the drawing aids window where you can check your snap settings. Make sure the snaps you wish to use are checked and make sure that No Snap is unchecked.

Also you can turn on “Turn "No Snap" mode off if any snap is activated” (at the bottom of Drawing Aids window).