Storage of deeds like wills is a very important matter, they should be kept in fireproof storage the person whose will it is instructs them to do this.
I'd ring them again and ask them to let you know urgently where the will is, ie did they send it to you or retain it for storage.
If they sent it to you then check the address they sent it to, address mistakes can be made especially when a sale of purchase of a property is involved.
They should also have an archived file, with no deeds on it, they must be separately stored, which can be destroyed after a minimum of 6 years.
As said above, you can revoke the former will by words in the new will but if they have lost your original will then I'd say it's pretty good grounds for asking them to do a new one free of charge or at least at a discount.
If they are being difficult or cannot provide a good reason why they do have it then ask for them to invoke their complaints procedure, all firms must have one, and if still no joy that you will take advice from the Solicitors Complaints Service.