This is often caused by another (non-Xert) ConnectIQ data field trying to access background services, which causes the display on our App to freeze. Note that the data recording is unaffected - only the screen is frozen.
To resolve this, you can try a couple of options:
Create a new Garmin Profile that doesn't have any ConnectIQ data fields installed. Switch to this blank/empty profile before launching the Xert Workout Player.
Alternatively, you can remove any non-Xert ConnectIQ data fields from your Garmin device. After removing non-Xert ConnectIQ data fields, try rebooting the device and then launching the Xert Workout Player