Hi, welcome to Just answer, my name is Ross, i will help with your question
This may be a silly question, but has there been any obvious signs of leaks?
Ok, the heating will blow cold as soon as the level drops to a certain point, this is because the coolant can no longer be pumped into the heater, this is a normal side effect of low coolant level, for it to have dropped this low, it would have either lost a lot of coolant very quickly or has been gradually loosing it, Although there may not always be obvious leaks, often it can drip when driving when the engine is hot and stop as soon as it cools down, this coolant often evaporates on the hot engine so leaves no signs.
It is possible, however because the level was so low simply putting water in there will probably cause air locks, this will cause it to overheat, its vital the air is expelled from the system. If it over heats during this process, we can move on to the next step. Is there any signs of water in the oil ( white gunk on the filler cap or dip stick) or oil in the water? (oil slick on the surface of the water in the tank)?
Ok, this is a sign of head gasket failure but its not conclusive by any means. First, ideally you need to examine the underside of the car, starting with the radiator and all of the pipes and connectors to the engine head and block and water pump for any signs of leaks, the system should be pressure tested so if water is escaping from somewhere it will show up. If this does not show any obvious leaks it could be the head gasket, this can be checked using a block or sniff test which tests for signs of emissions in the exhaust, also a compression test may be necessary, this is advised to be done before diagnosing a head gasket due to the cost of the repair. Common causes of this are leaking radiators or water pumps, if there is nothing obvious it could simply be a faulty radiator cap, these contain a pressure release valve which if becomes faulty will allow steam to gradually escape and the level to drop.
A garage will have the correct equipment to quickly and positivity identify the problem so is the best way to go. You can do the basics your self by getting under the car as well as you can with a torch to look for visible leaks. Check for signs of white gunk on the dip stick or oil filler cap. Excessive white smoke from the exhaust when the engine is hot
If its over heating, i wouldn't advise it, the engine will reach damaging temperatures very quickly but the over heating may be due to air in the system so if you can, remove the filler cap, set the cabin heater fan on full and set to HOT, start the engine with the cap off and let it idle, if you can, gently squeeze the radiator pipes, this will help push air out, you will probably notice the level in the tank drop as air bubbles come out, top up as necessary, then replace the cap, its vital you get hot air back, as soon as it starts blowing hot in the car, you can keep this blowing hot, this will keep the engine cool, providing the temp gauge does not rise beyond normal, it will be fine to drive.