First, please, try relogging into your Ultimate Guitar account.
If that doesn't help or you don't have an Ultimate Guitar account, please reinstall Tabs&Chords app using these instructions:
1) Delete app and log out from all of your Google Play accounts on your device.
2) Go to settings-> Applications-> Google Play Market 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 install Tabs app. All your in-apps that you purchased previously will be restored.