Apple Watch side button not working

Which Apple Watch do you use?

Did you already restart your watch?

If this does not work, setting up the watch as a new device would be a way to rule out a software issue.

You will only lose your created Watch faces and may have to activate the cards in your Wallet app again.

Then test the Side button again.

Unpair and erase your Apple Watch - Apple Support

If still no luck, this is hardware related and your watch has to be serviced:

Apple Service and Repair for Apple Watch - Apple Support

Then make an appointment at the Genius Bar or take it to your local authorized service provider and have it looked at.