Accessing External Sharepoint Site

I am able to access internal Sharepoints within my company and have up until now been able to access Sharepoints outside the company.  When I now try to access an external Sharepoint, I get the following message on the external company's landing page.   

"Your account has been locked. Contact your support person to unlock it, then try again...."

The external company states they have not made changes any access protocols.  Likewise, my company says nothing has changed with respect to the rules/ability to access external Sharepoints.  Someone indicated that the Microsoft “federation”  settings/setup of the two companies may (now) be incompatible.  Has anyone encountered this issue?