Torque converter clutch circuit is stuck on, what's the problem with the transmission and how expensive is thi?

I was driving my 2000 Chevy Lumina and realized something happened to my transmission and couldn't go past 45mph. I got it to a parking lot, my husband came and said he thought it was drove fine. He changed the oil and used his "reader" and it's code said there was a problem with the "torque converter clutch circuit is stuck on". How big of an issue is this and how expensive?