Without more details about what shooting modes and settings you have selected, your question doesn't give us very much to go on.
The 70D is a highly configurable camera and with some combinations of settings selected the behavior you describe would be as expected. It is possible that via certain custom settings options the AF has been disabled with a half press of the shutter button and requires you to use the AF-ON button on the back of the camera. It is also possible that via other custom settings the function of the AF-ON button has been reassigned to something else. If you are not intimately familiar with the various settings and menus of the 70D then perhaps you should try to reset the camera to the default factory settings and see if that solves you problem.