That doesnt sound too good I am afraid. I'd wait until late spring to see if there is any new growth before taking it out. Scrape back a bit of bark with your thumb nail, if the wood is green underneath then it is still alive.
As for causes there are several;too dry, too wet and then damage at the roots. sorry difficult to diagnose.

Hi Howee...
My Clematis Armandii develops quite a number of brown leaves which drop, but it produces young new leaves too.

Can you can see any new shoots ? If you can, then your Armandii will probably recover.

Ok thanks - maybe the dryness last year? Does the wind bother them? I'll check it properly but the whole thing is brown

That's a pity if the whole climber is brown.
Look very carefully for new shoots...
... and, unless you need to remove it now, you could give it a chance and wait a few weeks ...

I'm guessing the dryness last year might have done more damage than wind, because my clematis gets blown about a lot and doesn't seem to mind ..

Ok i wont give up yet, it was the first thing i planted!

If there's no frost due, water the ground around the Armandii with rain water. The UK has a mild week forecast, so you could probably do that today...

I'm not sure where your garden is in GB ...
Is your Armandii on your GoY photos ?

fingers crossed for you...
I understand how you feel about the Armandii ..
I get very attached to special shrubs I've planted ..

I'm in Newcastle in the North East, its very mild today i'll take your advice. No, no photos of the clematis

Good luck. Let's hope the combination of a mild week and the rainwater encourages your Armandii to put on new growth :o)

Howee, mine sounds like yours. All the leaves are brown. Its on the south facing house wall though but I suspect its probably a bit too windy where it is. It pulled through last Winter's horrendous conditions .... just. I cut it back to ground level last Spring and it grew again. As this Winter has been milder, I'm disappointed its brown again, think I'll move it next month.