Would you please try accessing the assessment link from a different browser or try clearing your browser cache memory? Try closing the browser and then reopen it and see if that works.
Do you see the same issue in other browsers as well? If yes, another reason might be there has been a long-time period of inactivity and you may have been automatically logged out. Due to this, you may not be accessing the link with your account. Would you mind logging out and logging in again to see if you are able to successfully access the link?