Troubleshooting a lens:

Lens

• Toggle: Make sure the toggle on the lens is firmly in the AF position.
• Contacts: Check the metal contacts on the lens to make sure they are clear of debris and are clean.
• Ring: Make sure the focus ring isn't obstructed. It should move freely when on MF and become more firm when on AF (for your lens, not all lenses do this).

Camera

• AF Setting: Select center point focus (this is your most accurate point of focus on the T3i) and attempt to focus via viewfinder)
• If you have a confirmed focus (beep or solid red light in the VF), then the problem is in the camera.
• Attempt to focus via live-view. If this works, then the problem is in the camera's phase AF (live-view uses a different, more basic system) and the lens and contacts are fine or the shooting environment.

Environment

If the camera legitimately cannot find a focus when on AF mode, it will not fire. This is usually caused by the camera not being able to determine distance to the subject. A solid color wall, or a very dark environment can cause this. This can also be caused by being outside of the focus range of the lens (usually too close).