If you are asking about Comcast's webmail ("connect.xfinity.com"), several items in the HTML toolbar including "Link" are not available if the compose screen is not "maximized". See the images in https://forums.xfinity.com/t5/Email-Web-Browsing/Appsuite-webmail-v7-10-0-1-3/m-p/3291863#M214496.
I am having the same problem. When I click on the screen maximize button, the email draft moves but does not expand and I still cannot see the hyperlink button. What else can I do?
After you open a Compose window try hitting the ctrl key and the minus [-] key [at the same time]. See if that helps bring the hyperlink icon back.