1994 Pontiac Sunbird overheated suddenly and died

You really need to list the engine size when talking about an engine problem. Do you have the 2.0L engine? The first suspect would probably be a jumped timing belt although that alone shouldn't cause overheating. A leaking cylinder head gasket can cause overheating but usually not hard starting. Do you know if engine power seemed normal or low after she got it restarted the first time?

Low power is a sign of a jumped timing belt. The belt on your engine runs the water pump too but it has teeth on the sprocket. That means it is not going to slip unless the teeth on the belt are worn off. I think I'd look at the belt first and check the tension on it and the teeth.

Depends on what "slushy" means. Your mechanic probably interpreted that as not enough antifreeze in the coolant and it has started to freeze. If you mean it looks like brown mud, that is from engine oil or automatic transmission fluid mixing with the coolant. Engine oil can mix through a leaking cylinder head gasket. Transmission fluid can mix through a leaking cooler inside the radiator but that is much less common.

You still didn't say if it's frozen or just looks yucky. If it's frozen and turning into a solid, a higher concentration of antifreeze is needed, but don't waste your time doing that now. To accomplish anything of value, you would have to drain out about a half gallon of coolant, then add a half gallon of straight antifreeze, (and you'd have to be sure it's straight antifreeze. A lot of suppliers are tricking people by selling it "premixed" meaning you're paying for half a gallon of water). First we need to know the diagnosis, then if the repair involves major engine work, that's the time the coolant will be drained and replaced. No professional would try to reuse the old coolant after doing engine work. That is especially important on GM vehicles. Acids build up in the coolant over time and that leads to corroded and leaking heater cores and radiators. New antifreeze has corrosion inhibitors and other additives that wear out in about two years. That's why we replace coolant every two years.

How long was she driving when it started to overheat? When the mixture is low on antifreeze, it will not freeze into a solid block of ice. It will be slush and may not circulate. The clue would be a really hot engine but the radiator would be cold. That overheating is going to occur within a few minutes of starting the engine. If she was driving 20 - 30 minutes or more before the overheating started, the coolant would not be frozen. Then the problem would be due to something else, like the leaking head gasket.

If all that's wrong is not enough antifreeze in the coolant, park the car in a garage where you can drape a blanket over the hood, then run the engine for two or three minutes about every 20 minutes. Don't let it overheat, but doing that should melt whatever is in the radiator. Once the coolant is flowing freely, drive straight to the repair shop, or at that point you can drain the coolant out and put new stuff in.

You'll never get all of the old coolant out when you drain the system so when you put the new stuff in, go a little heavy on the antifreeze. I like to drive the car to mix the coolant, then I test the freeze point, then add water or antifreeze as necessary to the reservoir. GM even made doing that unnecessarily difficult by forgetting to put a radiator cap on most of their products. The best you can do is leave as much room as possible in the reservoir so you can add what's needed later. If you don't have a freeze tester, any repair shop will have them, and most mechanics have their own. You will likely be able to have a mechanic take a minute of his lunch break to run out and check the freeze point for you. It only takes half a minute.

Also be aware that GM has been using red Dex-Cool antifreeze. We call that "Dex-Mud" because of all the problems it causes. If the cooling system is completely flushed, the more common antifreeze can be used, but the two types can't be mixed. You have to get all of the old stuff out if you switch to a different kind.

When there's no radiator cap, you have to open something on the engine to let the air bleed out so the coolant will run in. Follow the upper radiator hose to where it attaches on the engine. There will be a bleeder screw or there will be a sensor nearby that can be unscrewed.