I think you need to provide a bit more information on what it's doing. Does it not even try to focus? Or does it try, but not achieve focus?
This is basic, but did you check to see if the AF/MF switch on the lens is in the MF position?
Is the lens rotated and "clicked" into proper position?
If this is an old lens, did you try cleaning the contacts on the lens? (You can do this carefully with an eraser)
Which specific lens are you using?
Lenses which have auto-focus also have a physical switch on the side of the lens (labeled "AF / MF"). Make sure it's in the "AF" (auto-focus) position.
The camera's phase-detect focus points wont be able to achieve focus if your f-stop is above f/5.6. If you're using an f/5.6 lens with a 1.4x extender then that lens would not be able to focus (but an f/4 lens would work). A 2x extender shouldn't be used unless you have an f/2.8 lens.
If the lens is making no attempt to focus then it could be damaged or dirty pins. If you remove the lens you'll see a number of pins just inside the lens flange (on the body side) near the bottom. You'll see matching contact points on the lens itself. Make sure those are not damaged or dirty.
Can the lens focus manually? If you swtich to "MF" (manual focus) mode by using the switch on the side of the lens, can you turn the focus ring while looking through the camera and see that it is working? You want to make sure nothing is jammed/stuck and that the focus ring turns smoothly throughout the focus range (it should offer a tiny amount of resistence but that resistence should be consistent throughout the entire range of focus.)
Do you have any other lenses you can use to test whether the issue seems to be isolated to just one lens or does it affect all lenses?