Hi Charlotte,
This could possibly happen because of any improper use of JavaScript syntax by which your IE may behave indifferently.
A simple example as given in the below article,
http://blog.aresgrp.com/crm/post/CRM-Ribbon-Bar-disappears-after-adding-JavaScript-to-the-page.aspx
I would not say that this could be the cause but may just be a possibility.
BTW : Which is your IE version? and is it the same in the other working machines???
Is there any latest update rollup applied to either one of the client PC's?
Finding these details will help us narrow down to the root cause
Thanks!
Cheers, Francis Edwin
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