we need more details about the types of machines, operating systems, network topology, connections types. there could be 5million things that could have cause a sudden loss of connectivity with remote desktop. It could be the account you are using as well.
You are getting a specific error about your username or password. So there is the issue. While your username and password may be right, they may not be right for the remote computer. Computer's don't just deside to let a valid account not work. Either your info is wrong (caps lock on, num lock is not on, etc..) or the remote PC user or rights have changed.
Check the remote PC accounts again. Is this a computer on the domain? Is the right domain selected? Local PC account, domain accunt being used?
Could there have been a recent update on either computers that has affected the remote desktop settings? Is it possible that the server administrator has reconfigured the server to deny remote login on a workgroup that your machine would be categorized under?
There are many questions that remain to be asked, as the others have pointed out.