All of a sudden, I cannot open Mail by clicking on icon on task bar. I have to go to File, and click on new viewer window. Help!!

I have a MacBook Air, using OS X Yosemite, v. 10.10.5. I used to open Mail by clicking on icon in task bar. Now there is a little dot below it, but I have to open it from File, New Viewer Window . I don't know what I did to change it but I'd like to go back to the original way. Please help!!

Also, now when I click on a link in an email, I have to close out mail to see the new page, which I didn't have to do before.

Thank you.