Error message when trying to compose or respond to email

Early this morning I was able to compose and send an email.  Since that time, no matter the browser/device when I try to compose/respond to an email I get the Error Message "An error occurred inside the server which prevented it from fulfilling the request."  First time I called Customer Service I was told to restart modem.  I did that and still received the message.  When I called again I was hung up on.  Very frustrating!