Unable to write anything in the content when click on the "New Mail"

Unable to write anything in the content when click on the "New Mail"

Hi Everyone, I'm sorry if anyone ever write this topic before and if its a duplicate topic.

My concern here is that when ever i want to write and reply a mail, i can't write anything in the content. Even my signature gone and it's totally blank. Click on the content table below, still it's unable. What should i do?