left mouse button won't work on 2 different mice

I have an iMac 27" 5k that i bought in December 2014. All software is up to date. My left mouse button stops working intermittently. It happened a few months ago and I thought my mouse was bad so I went and bought another one. It did the same thing. After rebooting numerous times, it just all of a sudden worked.

This time it has stopped again, I reset PRAM (suggested in another post) and it still didn't work. I have rebooted many times and it still will not work.

Hope someone can help.

Debi