Left-click on mouse isn't working - 2008 MacBook

The left-click on my Macbook's mouse stopped responding, and I encountered the same issue when I plugged in an external mouse. I have been searching for hours for a reason and solution, but most responses I've found to the issue do not apply in my case (ie: I have never used a wireless trackpad). I did have my batter and power cord replaced about a month ago, and everything seemed to be working fine.

If anyone has suggestions or ideas, please let me know. It's difficult to send multi-page reports from an iPad!

Thanks in advance!!!