Sometimes someone who has a right but not clear understanding what he\she is doing can easily close public web part on the page.
Maybe you were wondering how to handle this situation and how to find any missing, closed web part.
The answer is simple - add to the page url where you want to manage web parts the param : ?contents=1
Here we go - the management will appear!
P.S. Every time I discover that somebody closed the web part I don't remember how to restore it!!! As reminder for myself: Go to the page where the web part should be restored and click Edit Page , Add webpart -> Gallery->Closed webparts