Here are 2 possible solutions:
Option 1:
1. Click on the Apple icon on the top left corner of your screen,then App Store. 
2. Click on Store/View my account and log in with the Apple ID you used for the original
purchase. iReal Pro should show up under Purchased apps.
3. If iReal Pro is listed there, simply delete the app and install it again.
(Use Spotlight (magnifying glass on the top right corner) to find iReal Pro on your computer.
You can Command-click from the Spotlight results to find the location of the app and delete
them and empty your trash.)

Option 2:
It's possible that your song library got corrupted. Make sure you have a backup of your songs as you might lose them and you need to reimport them from the forum.
1. Use the Go to Folder command from the Go menu in the Finder and copy this:
and delete the folder: com.massimobiolcati.irealbookmac
2. After that, try to open the app.

If none of these solutions worked, contact us directly.