You can try these steps to see if they’ll help:

Clear the browser cache
Clear cookies
Refresh the page (F5)
May try Google Chrome browser to open
If the issue occurs during login, close your Internet browser window and log in to again.
If you’re typing/copying URL from a book or other place, then please recheck the exact words of URL and ensure that no letter is missing.
Even then after if problem persists, then please contact helpdesk.