My van worked fine teasterday why is it overheating today?

There are several possibilities that come to mind.

Coolant loss (IE) leak in cooling system

Thermostat stuck shut

Water pump failure

Thermostatic fan clutch failure

Radiator Cap failure

Collapsed radiator hose

Cooling passages obstructed

Dirt and debris obstructing radiator

Fan shroud, air dam, valances broken or missing