Just happened to me, after the Galaxy Wear app prompted for an update. This downloaded and installed a new version of the Gear S Plugin, which couldn't start.
Note: The following steps will reset the watch, so make sure you have a backup.
The solution was to downgrade Gear S Plugin to the previous version:
1. Uninstall Gear S Plugin from your phone and force close the Galaxy Wear app
2. Go to APK Mirror and download and install the previous version. The latest one, which was downloaded automatically by the update was 2.2.03.19022841N (March 12, 2019). I downgraded to 2.2.03.19010761 (January 22, 2019).
3. Put the phone into airplane mode. This will prevent the Galaxy Wear app to download the latest version again.
4. Fire up Galaxy Wear, which will start the sync process again with the watch.
5. After this, you should have a working setup.
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