A builder insolvency mid-build is horrible and survivable, and the first week decides most of what you get back. Verify it on Companies House, stop payments until you have advice, secure the site and your materials, and check card and finance protection before writing anything off.
For that last step: the cost calculator gives the fair range and the quote check the verdict per quote. Still trading but not turning up? That is the delays route.
Look the company up on Companies House (free): liquidation, administration and strike-off actions appear there. A builder who has stopped answering is a delays problem; a company in liquidation is an insolvency problem, and the two routes are different. Verify before you act.
Possibly, and this is the first thing to check. Under Section 75 of the Consumer Credit Act 1974, your card provider is jointly liable for breaches on purchases over £100 and up to £30,000 paid (even partly) by credit card. Debit card payments sometimes qualify for a chargeback instead. Call your card provider before you write anything off.
Not before you have advice. Money paid to an insolvent company joins the pot for all creditors; you rarely see it again, and it does not buy the completion of your build. If a liquidator or administrator is appointed, deal with them in writing.
Usually their contract is with the builder, not with you, and paying them directly without advice risks paying twice for the same work. Be sympathetic, take names, and check with Citizens Advice or a solicitor before any money moves.
If your builder sold the job with an insurance-backed guarantee or a structural warranty, this is precisely the event it exists for: the insurer stands behind the work when the firm no longer can. Find the certificate and claim early; and for the difference between paper promises and insured ones, see the guarantees guide.