Verify that the server address, port, username, and password are correct in Filezilla. If they are, check with your instructor to verify that the information you have is correct. You may need to temporarily disable or add an exception to any firewall and security programs you have installed.
Filezilla's output may be useful in diagnosing the issue. For example, if there's a problem connecting to the server you might get an error like this:
Status: Connection attempt failed with "ECONNREFUSED - Connection refused by server".
Or, if your username or password were incorrect:
Response: 530 Login incorrect.
Error: Critical error
Error: Could not connect to server
and so forth.