I assume you have a low enough elevation beyond the dam to run a siphon? If so, get one started ASAP to get the water off of the overflow pipe. Use the biggest piping you have available.

Link to video how to siphon with cheap 4" corrugated plastic pipe.

https://youtu.be/x4JmX4gHUiQ

Unfortunately, it will take 9 of the 4" pipes to equal the flow that is currently going through your 12". However, every little bit helps.

You must have a lot of water still moving through your watershed. Anyway to divert some of the flow upstream to go around your pond?

Hopefully, the experts can give you some advice to fix the problem after you determine the exact cause.