Was it running running hot at the time? IE was it overheating and the fan had come on?
Has the cooling system been touched recently?

If the cap's ok and it's been bled properly so no air locks you might be looking at something a bit more serious.
Make sure the water pump belt hasn't failed.
Thermostats can fail so they don't open, so might be worth checking.
The water pump may have failed (normally they have a tell tale leak below their location where you park)
Or possibly the head gasket failing and pressurising the cooling system.