1680 mainspring

[Nickn]

I was just setting my 1680 to gmt when I felt the main spring slip or break
But now the second hand won’t move when I gently shake the watch
Up until the spring broke it was keeping time to around 3 seconds a day
Before I send for a service
Any ideas why the second hand isn’t moving when shook

[Liga]

Very unlikely to break a mainspring when setting the hands.

Was it difficult to set the time?

[Nickn]

Sorry I had set the hands and was winding the watch

[Liga]

It could be many other things other than a mainspring.
Yes the mainspring is suspect in this case.
If the click failed or the click spring you would feel the exact same thing.
If teeth broke on the ratchet wheel, highly unlikely but it would feel the same.
There are other winding and setting parts that could have failed.

You cannot always jump straight to the mainspring. But it very well could be.