The Watch is if fully charged but when powering on it shows screen wuthering logo that says Rebooting..... and stays on for a few moments and then goes black, and cycles through the black screen and rebooting screen until the battery goes dead.
I'm so sorry to hear of what is happening to your Galaxy Watch! Let's figure out what's causing this, and get it fixed, so you can enjoy your device once again. When did you begin having trouble? Has anything happened that may have damaged the device and caused it to behave in this way?