Talk to your new solicitor.
Ask them to talk to your original lender. Has the original lender got the Deeds?
If yes , there is no problem.
If no, the Law Society will investigate. It is extremely unusual for a solicitor to simply close up without dealing with outstanding files - unless they were very high profile ,as per very recent cases.
If all else fails, it is possible to reconstitute title - this would not be that difficult here - if this is a new house and your builder's solicitor will usually oblige with copy documents if necessary.
But first talk to your new solicitor.
mf