'96 ford taurus freezeplug / coolant issue :(

I was driving home from Maine on x-mas day when my 1996 ford taurus wagon made a loud thud noise ( I felt something hit under the car ) like I hit something in the road and smoke started pouring out of the engine bay. Im almost positive I didn't hit anything in the road, but I can't be sure. The car didn't seem to have power loss and kept running fine, but i quickly pulled over and turned the car off. The engine temp etc were all normal and I assumed it was a radiator hose or something because I could see coolant pouring out from under the engine bay. I was able to get the car towed off the highway and had it looked at the next day, and the mechanic believes it is a freezeplug issue and that it appears I overheated the engine melting(?) the freezeplug. Does this seem possible / is it expensive to fix? My local mechanic will be checking it out soon, but I want to know what im looking at cost wise.

Thanks :)

Never heard of them melting, but I have seen a lot of corroded ones fail.
Cost- depends on the location and what it takes to replace. I had one fail on one of my cars. I used a rubber expansion plug to replace it, cost less than five dollars and took fifteen minutes to install. On another car, pulling the engine was required.