1. Network problem: There is a network problem on the website of National Open University, which prevents users from logging in. Users can try to change the network environment or use other browsers to visit websites.

2. Account number problem: the user entered the wrong account number or password, which made it impossible to log in. Users can check whether the account number and password are correct and re-enter.

3. System problem: There is a problem in the system of the National Open University, which makes it impossible for users to log in. Users can try to wait a while before trying to log in, or contact the customer service staff of the National Open University for consultation.