User login error on admin panel of site

I have a admin panel for register users, which has not been updated and the hosting company can not find an issue with the server, however some users 'out of the blue' can not log in and when I set up a new user I can not log in with the username and password too.? This project will be to update the admin panel to correc the error. Also in the administrative area of the admin panel is an area to upload videos to the site however I have tried to add new ones to the site and these are not uploading correctly either as they are not displaying in the player window. This will aslo need updating. regards, Aaron