Notes access errors come from its security model, which is genuinely more layered than most mail clients, so the resolution depends on which layer is denying you, checked in order:

The ID file and password, the foundation: Notes access is tied to your Notes ID file and its password, so an access error can be a wrong password, a missing or moved ID file or the wrong ID file being used. Confirm you are using the correct ID, File, Security, User Security shows the current ID and that its password is right, since Notes authenticates you by the ID before anything else and a wrong or missing ID denies access broadly.

The ACL, the database level permission: every Notes database has an Access Control List defining who can access it and at what level, and an access error on a specific database often means your ID lacks the needed access in that database's ACL. The database's manager adjusts the ACL to grant your ID access, File, Application, Access Control when you have manager rights or the administrator does it, since this is a permission the database owner controls rather than something you clear yourself.

The server and connection, for server based databases: a database on a Domino server needs your client connecting and authenticating to that server, so an access error can be a connection problem, the server unreachable or your ID not registered for that server. Check whether you can reach the server at all and whether other databases on it open, since a broad access failure across a server points at connection or authentication to the server rather than one database's ACL.

The encryption layer, the deeper case: a locally encrypted database or one with encrypted content needs the specific ID it was encrypted for, so an access error on an encrypted database in hand may mean you lack the ID that can decrypt it, the same ID dependency that runs through all Notes work. The diagnosis order is confirm your ID and password first, then the specific database's ACL, then server connectivity for server databases, then encryption for a database that resists despite correct ID and ACL, each layer being a different fix and the error message plus which databases work narrowing which layer is denying you.