replaced a radiator hose on a 1999 sable after overheating.Now white smoke and coolant are coming out of the tailpipe. How bad is it?
Very. I suspect your radiator hose burst sending coolant everywhere, and you drove it till the engine overheated. There is a gasket between the head and the motor called a headgasket that holds the coolant in. Yours is allowing coolant to go into the engine and out the exhaust. But I don't think just the headgasket is bad, due to the heat the head probably warped or cracked, allowing the coolant to bypass this gasket. So, in not so specific words, your going to need an engine rebuild or new engine. Sorry.
I forgot you might also have a cracked engine block. Time to have a local mechanic assess the situation and offer you some options.