There could be many reasons but the most probable is you have simply forgotten your password, are trying the wrong one or are entering it incorrectly. Some other things to think about include:
Does your username or password contain a mixture of upper and lower case letters?
It should be entered exactly If you do not remember your username and password, please contact technical support for assistance.
Locate or search for the desired course (you can click 'All courses...' in the 'My courses' block) and click on the course name.
To create a new posting on a forum, click "Add a new discussion topic." Similar to an e-mail, you will be prompted to enter a subject line and then compose your posting in the word processing box. If appropriate, you may also add a file as an attachment to your posting. Click "Post to forum" when finished with your posting.
This depends on how your instructor has configured the forum. In most cases, click on the initial post (in the Discussion column on the forum's page) to see the text of the post. Each reply is underneath the initial post to which the reply was made, indented to show the order of the threads.
Use the latest version of your Internet Browser (latest versions of Firefox, Google Chrome, Edge, Explorer, or Safari (Mac users)
Have access to and be able to read, compose and save files using Microsoft Word and Microsoft PowerPoint
Save your work frequently
Write down your passwords and save them somewhere private