It is normal for your Apple Watch to become disconnected from your iPhone if the two are taken out of range of each other. They should then reconnect automatically when brought back into range.
Check that both Bluetooth and Wi-Fi are enabled on your iPhone (Settings > Bluetooth / Wi-Fi).
Check also that Your Apple Watch is currently connected to your iPhone: Swipe up on the watch face, swipe left/right to the Settings Glance, check whether Connected or Disconnected (disable Airplane Mode if it is active).
Check that your iPhone software is fully up to date:
Update the iOS software on your iPhone, iPad, and iPod touch - Apple Support
Check that your Apple Watch software is up to date:
Update the software on your Apple Watch - Apple Support
Then try restarting - or, if that doesn't work, force restarting - both devices and see how you get on over the next couple of days:
Restart your Apple Watch - Apple Support
Restart your iPhone, iPad, or iPod touch - Apple Support