No Heat in '93 Cav.

Ok, last night heat worked great. Plugged in the block heater as usual, started my car up this morning and NOTHING but ice cold air ALL the way to work. Blows fine, but cold. Hoses are both HOT into & out of the hearter core. Upper rad hose was hot as usuall, flowing through as usuall.It did not overheat or anything. Ran fine all the way to work, just NO hot air.

Any ideas as to what I might check ?