There are a range of scenarios. In most cases, sole practitioners have an arrangement with a colleague to step in and take over, in the event of serious illness or death.
On retirement, it would be usual for a solicitor to notify clients that they are doing so and offering the client the option of collecting their will or having it passed to a solicitor of the client's choosing or to a nominated colleague.
In worse case scenarios, where the practice just "stops" and no-one is dealing with anything, the Law Society step in and contact clients.
In terms of wills and getting a bit groggy , its a good idea to have a copy of your will with your "top drawer" papers and also to give your executor a copy, in a sealed envelope, if you prefer.
mf