98 Lesabre Motor Locked Up

I have a 98 Buick LeSabre Custom with 130,000 miles on it. My experience with this car was fabulous until one day I drove it to the bank in the morning and it was fine. Then in the afternoon I got into the car and started it up. I noticed it was running a bit rough but didn't really think too much of it since there wasn't any unusual noises. As I drove it down the road it acted like it didn't want to take any gas as I was trying to get up to 40 miles per hour. I slipped it into neutral as I was driving down the road and gave it gas, it seemed fine. I put it into drive and again it acted like it didn't want to take any gas. Almost immediately the motor cut off, without any noises. All my guages looked normal just before this happened, I coasted off the road and checked under the car and the hood for any obvious explanations but saw none. I tried to start the car back up but it didn't want to start. It gave maybe a 1/4th turn then locked up. I had it towed to the mechanic who says the water and oil were clean. And the timing chain is not broken or out of place. He tried to check the torque converter but could not get it off because the motor completely locked up. The strange thing is that there wasn't any noises to explain why the motor would suddenly lock up. Does anyone have any suggestions or ideas? Any help would be greatly appreciated.