Serious problem with watch since software update

Hi,

I have a serious problem with my watch since doing the software update this morning.

Watch face keeps flashing between the Apple logo, clock face and passcode.

Watch is unresponsive when you try to enter passcode and side button not responding so cannot power off to try and reboot.

I have tried to unpair the watch from my iPhone 6S but getting message saying 'Cannot Unpair Apple Watch - Activation Lock could not be turned off on your Apple Watch because of a server error'.

Nothing seemed untoward during the update, I just left it to do its thing.

Any help would be gratefully received.

With huge thanks

Nicola