I used Zwift, and now I cannot connect with my Stryd!

If you have the Zwift app open on any of your devices, it will try to connect to Stryd. Because Stryd can only connect to one Bluetooth device at a time, this can cause problems with the Stryd app, Suunto, and Polar devices. To avoid this problem, make sure to quit Zwift and / or switch off the Bluetooth functionality on the device you use for Zwift.

Note that Apple TV users have reported that when the Apple TV is in sleep/stand-by mode, it maintains the Bluetooth connection with Stryd. As long as that is the case, no other device can pair with Stryd.