heat works fine until I stop at red light or pull over then blows cold

OK. Changed water pump 2 days ago. Heat worked fine. Tues. night white smoke BELLOWING out of exhaust pipe. Went out to check fluid levels this morning, and van wouldn't turn over, was making loud grinding noises,,, I've had van for 60,000 miles and never once did it overheat right up till this morning. No water in oil, never lost coolant, never had a problem. Changed oil every 3000 miles, etc.etc. Motor is seized, isn't it??? How or why did this happen. Symptoms were SO sudden. Check engine lite never came on. What would van be worth for parts?? Any ideas????

headgasket . pull the sparkplugs . look for white ashy color on a plug or water running out of cylinder .