2000 Hyundai Tiburon Died

Great question! It sounds like your engine may have overheated or seized? Was the engine running when it suddenly stopped? When you tried to start it, did it make any noise? Would the engine turn-over, or did it seem like the engine was completely seized? Did you see any "wet" coolant on the engine compartment, or was it all dried-up? Having an engine overheat and seize can be caused by several things, but I would start with checking and adjusting the coolant and oil levels first. It is common to have some leaking coolant, because most vehicles have an "overflow" tank where hot coolant is stored until the engine cools, then it is sucked back into the engine. Sometimes this can overflow out of the overflow tank and get onto other parts of the engine compartment. Have you checked the level of coolant inside the radiator? Is it low? Have you checked the oil to make sure that there's no water in it? The oil will turn to more of a milky yellow color if water gets into it, which could be caused by a blown gasket. Also, if the water from the radiator leaks out and the engine overheats, it is possible that the engine seized-up. I would start with that first.

I was driving on the highway when it died. It would turn over but wouldn't start. It was towed and then started just fine. Went to drive it & it wouldn't go over 20 mph. Has trouble accelerating. It overheated when I hit 40-45. So I had to pull over and shut it off. What could cause this?

Sounds like you over heated it, and blew small portion of head gasket. -and/or warped the aluminum head. ...Cold the "hole" closes up some, but expands with the expanding engine components with heat.