If you are part of that it could pay to investigate that long lost passbook you found in a grandparent's chest. In the case of an investment or savings account it is a relatively simple matter of presenting some valid identification and nominating to either reactivate the account or take the money out. With regards to investments held by deceased parents or relatives, that money would become part of that person's estate, regardless of whether or not it was known that they had it at the time of their death. What to do with the money would then become a matter for the executor of the will. It is a similar case for shares.
Companies keep a register of all shareholders.
If you own shares in a company and have lost your share certificates then you should call the company itself and ask to speak to its share registrar. They will ask for some identification, probably photocopied documents, and will then check their records to find your shareholding. You will be issued with a letter of indemnity, which will serve as proof that you own the stock and will then be able to claim any dividends that you are owed. If you want to trade the stock after that you will need to get new stock certificates issued. This will cost you a nominal amount.