If your Watch is unresponsive when you try to hold the side button, it's time for a forced reboot: Simultaneously hold down the side button and the Digital Crown until you see the Apple logo (it may take up to ten seconds).
If that works, slide the Power Off switch to the right to turn off your Apple Watch, then press the side button again to turn it back on.
We also recommend checking the iPhone's Apple Watch app for any software updates you might have missed, as they often can solve bugs and improve performance.
If that doesn't work, try a force quit: Press and hold the side button on your Apple Watch until the sliders appear, then press and hold the Digital Crown until you return to the Home screen.
You can try fixing this problem a couple of ways: Press the Digital Crown to try and return to the Apple Watch home screen, then relaunch the app from that home screen.