MacBook Pro Left click not working on trackpad as well as mouse
I had the same problem. After logging in to Yosemite 10.10.4 on my MacBook Pro (mid-2014) the left click on the trackpad did not work. Also, when hovering the pointer over links etc. they were not clickable. Right click were fine.

Rebooting did not have any effect.

It was possible to left click on the Shutdown button in the login screen so the trackpad itself works.

The solution was to close the lid for ~60 seconds after logging in. Then it worked.