mac mail cannot enable existing iCloud m

Suddenly I do not have access to my iCloud mail account on my MacBook pro. I have already tried to go to mail > preferences > iCloud mail > clicked on enable and it does not work. I also went to system preferences >accounts>clicked to include mail. It requested that I add a cloud account. When I entered my traditional mammal name I get a message saying that name is taken already. I cannot just click on mail to activate or enable. In mail I tried taking all accounts off line and then back on line. No iCloud account. I am using a Mac Book Pro High Sierra OS