Configure the TS Gateway with error “the logon attempt failed”

I need someone to help me figure out why the TS gateway we have been using to enable our employees gain remote access for quite some time is no longer functional. All users trying to gain access through the TS gateway are denied even if the user credentials are right and the message “the logon attempt failed” is displayed. The employees have the option of connecting to a terminal server Pc or going to the company’s website and launching a TS remote application which are both routed through a TS gateway. The system has been running efficiently until today. This is a serious situation that needs to be solved urgently.
