You probably need the file in any event so the first thing to do is to find it. If the practice is still physically there and is being operated and in the absence of any notification to the client, it is likely that the file is still there and has either (a) fallen down behind a radiator or (b) is on someones to do list. If the file was never passed on to the new solicitor, then there needs to be an exchange of information/communication as to who has done/should do what and who has paid or is paying for what? It could be the case that the original solicitor had various undertakings outstanding which may have been taken over by remaining partners. It can be awkward enough for a new solicitor to take over old undertakings and can be better for the previous firm ( if still in existence) to comply with undertakings before releasing the file.
The new solicitor should be able to elicit the information required from the old practice and/or the client and/or the lender and, if all else fails, the Law Society.
mf