My activity app on my Series 3 watch quit working. Any suggestions

First try force restarting your iPhone by whatever method your version calls for.  As soon as you see the Apple logo force restart your watch by holding in the crown and side button for 10 seconds till you see the Apple logo then release.  Log into your iPhone and wait for your watch to boot, then log into it.

Failing that I'd try unpairing the watch from your iPhone then pairing it back setting it up as new.