Under US law you can be compelled by a court to unlock (provide passwords / decrypt) a device if the contents of the device are known (the helpful search term for this is forgone conclusion). Otherwise it should be covered under the 5th. As for "forgetting" a password in this case, you will be held in contempt of court if they don't believe you, which can mean indefinite detention.
IIRC there are some historic high court rulings setting a precedent that detaining a defendant for not furnishing some information (combination to a lock) violates their protection against compelled self incrimination / right to remain silent but if it was a physical object (key to a lock) that did not necessarily voilate same rights. This has now led to digital age cases where the prosecution suggests a password should be analogous to a key, not an idea. On a quick search, I couldn't find the relevant cases or discussion...
