When your app misbehaves (and don’t we all from time to time?) it may just need to be restarted. Just force close MyFitnessDJ then restart from your home screen. Restarting gives MyFitnessDJ a fresh start and may solve your problem.
If that doesn’t help, try a hard restart of the device, which lets the device restart with a clearer head and ready to behave.
If these steps don’t help, check out the MFDJ FAQs to see if your problem has already been addressed.