First, please, try to relog into your Ultimate Guitar account in the app's Account&settings menu.
If that doesn't help or you don't have an Ultimate Guitar account, please reinstall Tabs&Chords app using these instructions:
1) Delete the app and log out from all of your Google Play accounts on your device.
2) Go to Settings, Applications, find Google Play Market there and make "clear data"3) Next open Google Play Market and log in with the same Google Play account that you made your purchase, and then install Tabs app. All the in-apps that you purchased previously will be restored.