Try running a recover on the file, but pray that it doesn't find a problem. File damage associated with account security usually cannot be repaired.
The obvious thing to check is whether you are actually entering the password correctly. Once upon a time, passwords were not case sensitive, but in today's versions they are. But I'd think that you couldn't open the file with a full access password at all if this were the case. What should work is for you to enter the precise account name (this is NOT the user name) and password that you used to open the file and access manage | security in the first place.