Gathering Information
Are you able to access the web interface using a secondary browser?
Are you able to access the web interface from a secondary workstation?
Do remote users have the same difficulty?
Have you restored web-admin access?
Secondary Browsers
It's often good to have secondary browsers installed to troubleshoot browser related issues. If a secondUsing the arrow keys, ary browser works for accessing the PBX then consider clearing the cache on the original broswer.
Other Workstations
Testing from a secondary or remote workstation will help us establish weather you, your IP range, or everyone is effected. Sometimes individual IPs can become blocked or access through the access control can be adjusted or removed causing a temporary block.Restore Access from the PBX
...