Had the same problem but these steps seems to have worked for me.
1. Connect to the charger - it was still stuck in the boot loop.
2. Hold the crown/button down for around 30 secs until a long 2-3 sec vibration.
3. The watch was able to boot after this. I went to setting on the watch and did a 'disconnect & reset'
4. Set it up as a new watch
Hope this helps.