So hopefully it’s just a mistake and they’ll reinstate it when you call them.
If they won’t reinstate it, then (as I expect you know!) you are still legally obliged to complete the purchase.
So in your shoes I’d call a mortgage broker - they will charge you a fee (I paid our last one £250, so not huge in the scheme of a property purchase) but they can arrange a new mortgage much more quickly than you’d be able to alone.
Also inform the whole chain what has happened and that you are urgently trying to deal with it but may have to delay completIon, very sorry etc. That gives them warning so they can try to reduce any expenses caused by a delay.
Is completion in one week? It’s unlikely they could get the new mortgage in place that quickly, so when the completion date comes the seller will serve a “notice to complete” on your solicitor.
You then complete as quickly as you can, but of course you’re liable for expenses caused by the delay. The other parties do have a responsibility to mitigate those expenses (ie keep them as low as they reasonably can).
If the mortgage was cancelled due to a mistake by the mortgage company you can complain to them and may get compensation for any extra expenses, if it was caused by a mistake by your solicitor (doesn’t sound like it could have been) then you can claim compensation from them,
Also worth checking if you have legal cover with your home Insurance as that may cover extra advice needed on this or extra legal fees eg around the notice to complete.
This whole mess is another really difficult flaw in the English property buying system! Must be very stressful for you, I hope it works out.