Don't worry. Each Zing comes with a "bootloader" firmware that will restart the device even if the main firmware becomes corrupt. Here's how to recover your Zing.
1. Unplug from outlet, let the internal battery drain completely -- usually takes between 24 hours.
2. Plug the Zing back in
3. A card named "Unknown Zing" will popup in the iOS app, click on it to resume and complete the update process.