Can't click with trackpad or mouse (OS X 10.9, MacBook Pro)
Solution 1:
One of the most common causes of this is a magic mouse turned on in your laptop bag or somewhere similar.
Turning off bluetooth will ensure no remote button is being held down.
Solution 2:
I just had this problem and was resetting SMC and all kinds of other things. Then realized I had set something on my Magic Trackpad under my monitor, which was registering a click obviously so killed all other clicking. Make sure you eliminate that possibility!