Mine is slow Silvermist its now just starting to show shoots.

It may just be late to grow.

It is almost impossible to kill Buddleja by cutting it back hard, just be patient.

Buddlejas are normally very robust and as a rule always respond well to hard pruning/ renovation pruning, it has been observed over many years that a buddleja pruned hard in the autumn can succumb to severe prolonged cold snaps, however on some occasions you may get some bud break near the base, if your buddleja appears to be dead do a few bark scrapes and check for healthy wood, the best way to deal with a buddleja if it needs a good prune is to come down a little on the top growth in the autumn, this will help protect it in strong winds as well as wind rock, this done then mid April if weather allows then adopt hard pruning/ renovation pruning, you will have by this time lots of young shoots all over the plant, prune out any dead whispy branches, i always prune my budlejas this way and results have always been good.

scrape a bit of bark off with your thumb nail. if its green underneath it is alive. if its brown then try lower down etc. brown/beige means dead sadly.

I lost several buddleia's in customers gardens this year that had been happily hard pruned in late February in previous years.

I guess it was the long period of cold miserable weather in March that did for them. Odd though because others pruned in exactly the same way at the same time survived. Must be the exact position and soil conditions that makes the difference.

Unfortunately if they're not showing signs of life now they've probably died

There is quite a bit of die-back this year thanks to the March snow. The green under the bark layer is a good test but much of the top growth could be dead. I would wait just a little longer (into early June) to see if it springs from the roots/ground level as they are able to do this even if all the top growth is dead.
But you may have to buy a new one. There are many great cultivars out there, some hardier than others but there is no general rule with B. davidii. I would recommend West Hill for a lilac, Blue Horizon for blue, Darent Valley for white and Pink Delight as the best pink. It is really worth seeking out the less well known cultivars - don't settle for the common ones.

Thanks for al of the replies and suggestions! I will wait til mid june at least before removing it and will try the bark test as well. If I have to replace it , it will be with another buddleia. It is so good for the butterflies. Lovely scent as well. Hope this one is still alive! Will let you know.

Oh dear. I've scraped the bark and it's brown underneath. :( I'll leave it be for at least a couple of weeks and see if there is any sign of life.