When you copy and paste from Word, you are copying all of the styling that Word provides for you. Word uses a special form of HTML code in its documents and unfortunately some of this code can break your website.
As a result, copy/pasted content from Word can cause your site to become disabled and in-operable. Your best bet is to copy and paste into a general text application like Notepad, then copy from that document into the ISM. Doing this will strip any unnecessary code that can break your site.