see volvospeed/bay13 for info on reading codes, or a Haynes manual for the 850.

A completely blocked exhaust would stop the car from running but this is very rare. As cars age or after an impact like hitting a rock, sometimes a baffle in the muffler or an internal part of the catalytic converter falls loose and restricts the exhaust. The car will then lose much of its power, especially when hot. The exhaust system will become very hot at the restriction. Hard to imagine a complete block that would make the car not run at all.

Do you think the no-start is directly related to having washed the car? Did you wash under the hood and dislodge some wiring? Is there any history of recent work or starting problems - where something that was disturbed was not put back properly and fell loose? Can you hear the new fuel pump run - that is, are wiring, fuses, relays intact?

Regarding the cam position sensor it is hard to see how there would be any spark if this is not ok.

Assuming the starter turns over but the engine will not run, you might try disconnecting the air flow sensor (on the inlet port after the air filter) to see if the car starts - if the sensor is bad it can cause non-starting. With sensor disconnected the engine controller should enter a "safe mode" where it will idle but will produce very little power (fuel mixture very weak). If you disconnect it a trouble code will be set though, even if the sensor is not actually bad.