Hello H.U.,
I’m very sorry about this — The current App Store version of Chordion no longer works on 5.1 — do you have a backup of your iPad? If so, restoring the backup should bring back the previous working version of Chordion.
If you end up getting a newer device at some point, please let me know and I can send you a promo code for one of our other apps.
Best,
Ben