The most common problem with the Autoharp app is no sound when you strum. The most common solution is to restart your device! To restart, hold the power button until a red slider appears, then slide to turn the device off. After it shuts down, press and hold the power button again until it starts up.
If this doesn't solve your problem or you experience a different problem, please use the help link above. For the quickest response, be sure to describe the problem in detail and state what kind of device you're using (e.g., iPhone 4) and what version of iOS you have installed (you can find this from the Settings app under General > About > Version).