Login to your Mendix account. Go to developer portal and check list of apps on your account. From there, select your app and click the option “Edit in Studio Pro”
If you have the correct Studio Pro version installed in which the app was created, it should open the app in that version. 

Another way could be opening Studio Pro and then from “Version Control” tab, click download from version control server. Then select your app. 

If still you are unable to access your app, you can contact Mendix support and provide them details of your app and how you are trying to access it. 
https://support.mendix.com/