You should submit your application, with as many details of the denial as you remember.
You are not the first person who has mislaid, never got or plainly thrown away the denial letter. It is quite a normal thing even.
As long as you are open about the fact you got a denial, and tell when (as good as you can) so they can check back if they feel the need.
In 10 years your circumstances will have changed and the visa officers know that. So they will have to look at your application as a new one, not based on the result of what someone else decided in the past.
One thing they will be sharp on, if you are caught on a lie, it will be much harder for you to ever get a visa again, as they do not accept people who lie.
So tell the truth, but also tell that you do not know, if you do not know anymore.