Whatever has gone wrong, it won't be anything to do with the fact you bought your subscription. As others have said, it sounds like perhaps SkyDemon no longer has the required permission to access your location data. Or perhaps if other apps don't work either, there is a more widespread issue with your GPS receiver.

I use a natty app called GPS Data to check the status of my internal and external Bluetooth GPS. Get it on the App Store....

Have you also checked whether location services is enabled for EACH app you want to use GPS on ... Go into Settings>Privacy>Location Services and make sure it is switched on for Skydemon and any other app.

This is what GPS Data looks like...

My suggestion would be to reinstall SD and check again. If it still doesn't receive a GPS signal I would check with other software to verify, that the GPS is working at all.

Dear Aviators,
I tried the trial version and was very happy with Sky Demon. It worked fine. This morning I bought the full version and went flying. I just saw 'Waiting for location data' all the flight. I use an ipad mini. Location services are turned on etc. I also notice my 'Maps with Me' has stopped working too. What has the paid for version done to my ipad that looks like my GPS doesn't work anymore?
Terry