Why does iphoto open when I try to open aperture?

I have been using Aperture for years but am not a technical user. Today when I click Aperture it opens iphoto library. It still says Aperture at the top of the screen but it's the iphoto setup. I am using a MacBook Pro Version 10.7.5. I haven't made any recent changes. I did do a complete back up onto an external hard drive the other night but haven't checked to see if that is okay. Looking for first steps (in non-tech language). Sorry if this is something basic. I just not sure how to proceed and can't find an answer.