Even in a NEW aircraft, it is never effectively sealed. In other words, it will leak and depending on the inherent leak-rate and altitude, this will determine the level of the emergency. In all likelihood it will cause an emergency descent but of course this rather aggressive maneouver can be modified to suit the cabin depressurization rate. In simulator scenarios, I have seen anywhere from 500fpm to 2000fpm cabin altitude climb rate, solely based on losing pressurization capability. (ie, no explosive decompression, faulty outflow valves, or hole in aircraft). You have to get below roughly 14K' before the O2 masks auto-deploy in the cabin. Then you need to get to 10K' for any sector over 30 minutes.
It is not the lack of O2 that will get you, it is the partial pressure of the O2. There is probably enough oxygen in the cabin, but actually getting it to pass from a person's lungs to his bloodstream, is the problem. A certain amount of atmospheric pressure is required.
Ram air is recommended in most circumstances below 10K' or less than 1psi differential pressure. The outflow valves should then partially open to allow for adequate airflow. Most large aircraft have a ram air inlet for such situations, as well as smoke removal procedures.
Oh yes, you asked if cabin temp will rise or fall. It will fall. Since pressure decreases, temp falls... just like in the earth's atmosphere. Naturally, it will fall anyhow, since at altitude temps can be as low as -72C (coldest I've seen). Even at 10K' in a standard atmosphere, the temp is -5C, so with the Ram air inlet open, it will get cold.