I am unable to get a response from the email or finder icon on the dock

I am unable to get a response when I click on the finder or email icon on the dock of my iMac. I can get a new finder window when i open the finder icon and click on 'new finder window' but this does not work with the email icon. I have checked the 'open at startup' when I open the icon, but if I close it then it won't work unless I restart the computer.