Two things: either deeds are lost irretrievably or they are mislaid. About 99% of the time, they do surface ( i.e. only mislaid) but its impossible to give a time line - anything from a few days to a few months.
At least if its acknowledged that the Deeds are lost, you can start the process of dealing with the problem. If lost, the title will have to be reconstructed and much then depends on what constitutes the title. Reconstructing the title can take a few weeks or a few months - its very dependent on the actual circumstances. You'll also be looking at title insurance. If your purchasers want a quick sale, you probably won't be able to do that.
The lending institution have to fess up - ask your solicitor to put a stiff letter to them asking them to confirm the position, advising that you may lose this sale, asking for their proposals to deal with the issue and confirming that you will hold them responsible for any any losses incurred. When this has happened before I find that some lenders will get right onto it, fess up and sort out. Unfortunately, some simply go into total denial - very frustrating. This leaves you and your solicitor in the unenviable position of trying to move something along with very little assistance. However, you are legally obliged to do what you can to mitigate your losses.
mf