I am no expert in this, but here goes....
I would not continue sailing unless there was no other option. Clearly with the spreader broken, to upper part of the mast is not fully supported, and you would risk putting a lot of stress on - and potentially damage - the rest of the rig
. Logic tells me you could get away with it in light winds, but why take the risk?
As for next steps, I guess the big question is why this happened. Was the spreader simply rotten, or is there a problem with the rig
? I'd get an expert in to look at the rig
before I went up to examine the spreader. Chances are it will be sound enough for somebody to go up and have a look, but you may have to pull the mast. Get/make a replacement spreader (they are normally fairly simple things), and then re-tune the rig
. Obviously inspect the "sound" spreader while you are at it!