Can you be compelled to provide something that you don't have access to? Were anyone else in this situation, wouldn't it be plausible to simply claim you don't know that password?
1) I use a password manager so I don't know the password. However, I have the means to acquire the password.
2) I use a password manager but somehow lost access to it unintentionally.
3) I use a password manager and lost access to it by design. (eg. Using a dead man's switch of some kind that deletes it if I don't "check in" for some period of time)
4) I used to know the password. However, I suffered a traumatic brain injury and cannot recall it.
I obviously don't have the answers but I think these are interesting to think about as different points in a large legal grey area.
If there is reason to believe that you are telling the truth (say, you're being asked which of two parking spaces you saw a car in 10 years ago) then you're fine.
Same thing goes here. Rational disinterested people (a judge or a jury) will look at the available evidence and make their best judgement about whether you are telling the truth when you say you can't remember.