I've been playing for 4-5 years or so.
I've practised and played a lot during this time.
A couple of nights ago I was playing for an hour or two and everything was working very well. Embouchure working very well. Hard low D easy, et c.

Yesterday evening something had happened. The lowest notes were very, very weak, especially d and e. And sometimes f# and g. Excersises that had worked very well before, no longer work at all.
The second octave works ok, though.

This morning it was the same...

If I had a gig today I'd have to ditch the flute totally and just use whistle in stead! It's that bad!
This is really worrying and I have no idea why it's happened.

BTW it's not the flute. I know this can sometimes happen when there is an air leakage. Not so in this case.
It's the same on all my flutes, though some work a tiny bit better.

It's something to do with the embouchure.

Ever had this happen to you?

Nope. I'd give it a couple of days and see if it continues. Keep us posted.

Yes. I assume you've already checked for leaks?

If no leaks, then I'm loth to point out years in the trenches because everyone's different, but although my embouchure was serviceably good enough at 4-5 years, it was only so for the most part, because every now and then I'd lose my way somehow. Sometimes room acoustics had to do with it. Sometimes all I had to do was play some backup, then go back to flute, and suddenly my lips weren't working. Try to figure that one out. Sometimes I couldn't point to anything at all. What it says is that while my embouchure was a good one, I had yet to actually master it, which is a different thing altogether. FWIW, it took me about 20 or so years to get to the point where I could comfortably permit myself to believe I'd finally gotten there.

At 4-5 years, I'd suggest that the occasional embouchure malfunction - heaven forbid - is always going to be a potential part of the landscape.

_________________
"Time is the wisest counselor of all." - Pericles

"I remain not entirely convinced of it." - Nano

Sometimes playing notes way up into the third octave can refocus a tired embouchure. You could give that a go.

Or you might find something in http://www.mcgee-flutes.com/Getting_the_hard_dark_tone.htm useful.

It seems unlikely that several flutes would suddenly develop problems, but I'd still check for leaks (suck and blow test on each section). And check the location of the stoppers. Just in case!

Several flutes could develop problems simultaneously if the are all exposed to the same sudden change in conditions, e.g. moving to a cold place with central heating. But I agree that it is the less likely cause.

I recently had a problem with my low D on my keyed flute which I eventually tracked down to a wobble in holding it. I was venting the EFlat key on all notes over D and lifting the finger for D - not realising that I had started to depend on the little finger for some of my support of the flute. This shifted my embouchure on that one note. Fixing the hold (i.e. concentrating on not moving the flute when lifting the little finger) on the flute fixed the problem.

Even on a keyless, if you are relying on the grip of the right hand fingers to keep the flute in place, or, alternatively, if you are holding the flute fine without them, but they disturb the hold once they are added... then that could be a cause of problems?

YMMV - may or may not have anything to do with your problem.

_________________
19th October, 2012:
Flute: Rookery
Flute: Musical Priest
Flute: Swinging on the Gate
Flute: Sally Gardens
4th June 2012:
Flute: Rolling in the Ryegrass, Green Gates
2 April, 2012:
Smallpipes: The Meeting of the Waters. Corn Riggs
Smallpipes: Mrs Hamilton of Pithcaithland

I like the room idea.

When I'm playing in my wonderfully resonant living room, I find it easy to focus the tone on my flute.

When I play outside or in a loud session, I can't match the tone in my head and try to force loudness, which just makes things go to hell.

But, Terry's suggestion on playing in the higher register has frequently helped me improve my low notes.

Sometimes a simple cause like dry lips can ruin my embouchure. Especially in winter with the dry air.

Thanks for your input, all of you!
Yes, I now realise this kind of embouchure problem can happen now and again to many players.
Room temperature and humidity may also have played a part at this time of year.

The problem got a little better after three days or so. And then I had an "emergency" Skype lesson with Brendan Mulholland. We went through his embouchure exercises and talked through the problems.
I worked some more on his exercises (as I have done a lot before as well). Two days later the problems were more or less gone!

The strange thing about it all was the difference of two mere days between everything working well and virtually nothing working. Quite scary.

BTW I can't recommend Brendan highly enough! He's such a great teacher!