Sorry to ask such a stupid question but the heater on my 2006 xc70 was working perfectly until today [when it snowed last night!] and now it only blows cold air around. Did I accidentally turn something off or on? It seems you have to be something of an engineer to understand the thing.

I try to just use 'auto' but not having any luck getting heat into the car right now.

I have a low coolant warning message constantly and I have been ignoring it but today I did seemingly get some overheating for the first time. Could be related to that.

Probably what we used to call 'finger trouble' when I was in the air force and a pilot snagged up the aircraft.

If your coolant is so low that your heater core stays cold, you have a serious problem. And it will get worse when the engine melts because of lack of coolant. You need to get coolant back in the system and then find out where it is leaking.

Besides "low coolant warning" should be in RED, capital letters, and flashing until fixed - if I owned a car company.
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Keeping it running is better than buying new

+1 on Chris Mullet's post.

I suggest you check for a leak where the heater hoses go through the firewall as mentioned in an earlier post by Will Rehsper. That can be a problem with these cars. I also suggest you go over the coolant tank thoroughly and look for cracks. Especially on the bottom! Look at the tank hose and all other hoses making sure they aren't cracked, deteriorating or loose. This is entirely possible due to the fact that any hoses that are original are now 15 years old. Plus your car clearly wasn't properly maintained.

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Will I buy another Volvo??? We'll see....

You're playing with fire Bob! Please sort your coolant issue before the engine is further damaged.

The lack of heat could be due to two things. Either the blend door is malfunctioning or there is no coolant circulating through the heater core.

The blend door(s) regulate the flow of air over the heater core. This is what alters the temperature of air out the vents according to the setting you choose. There is no valve that adjusts the flow of coolant, it flows through all the time - unless there is no coolant in the system...

Sure sounds like you have low coolant due to a leak, overheating, or a blown headgasket pressurizing the coolant system and pushing it out. How did it overheat on you? Remember that if there's no coolant on the temperature sender, the gauge won't read correctly and may not indicate overheating.

I suggest refilling the system, even with just plain water if you suspect it will lose it, then running car at idle to observe what happens. Does the heat in the cabin come back? Can you see any leaks under the hood or in the cabin? Pushing water out the expansion tank? Does the white vapor from the exhaust never go away even after it's warmed up? Might take a while (30minutes or so) to get there.

Best of luck,
Will
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XC60 / Odyssey

Low coolant level will definitely affect the heater performance. Don’t ignore that “Coolant Low” warning as it may cost you an engine some day. Apparently you’ve got a coolant leak somewhere - you need to investigate where it’s escaping.