help!
Pyracantha - we had a lovely pyracantha in a pot, this year it flowered amazingly, and had tonnes of leaves and berries. We were having a move round, and discovered it had massively rooted through the pot and the block paving! Very stupidly, I cut the root, fretting about the block paving, unsurprisingly, it has gone completely brown and all berries shrivelled. It has real sentimental value and I am wondering if we can save it?

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I'd get it out the pot, refresh all the soil, water regularly and cross fingers. Don't have it sitting in a hot, sunny site while recovering. There seems to be some other weeds in there too, so remove those too.
All that brown top growth won't recover and regrow. If you cut it back to about half the size, or to decent joints - hopefully with some green showing, it may recover.
The top growth is dead, so it's no use to the plant at all, and removing it, or a good bit of it, will simply make it easier for it to recover, as there's less 'plant' to support and keep moist.
Hopefully, it will send out some new growth on the remainder. The dead foliage will just drop off that
Each year, it'll need the medium it's growing in refreshed - that just means removing a bit of the soil on the surface, and replacing it with some fresh soil/compost. Any plant needs that if it's in a pot long term.
Ideally - they like a nice bright sunny spot to flower and fruit well, but they also grow perfectly well with a bit of shade. In hot weather, pots dry out very quickly, so a shadier spot helps prevent that, and it's certainly the best idea while it's recovering.