See how to restore a closed Web Part. If an end user has accidentally closed a Web Part he can restore it since it's not deleted.
There is a difference between a closed Web Part and a Web Part that was removed from a site. First of all if a Web Part is removed from a site it's not gone forever. Web Parts can be added and removed from sites and pages while still being available inside SharePoint and to other sites, pages and users.
If you add a Web Part to a site it's rendered and adds time to the page until completely loaded in your browser. If you remove the Web Part it won't be processed the next time you load the site in your browser meaning that the site will load faster (even if you don't notice it.
If you close a Web Part it will be processed but it's not shown on the site. So loading time in your browser is affected which is a problem if you close several Web Parts. At this point it's better to check for closed Web Parts instead of investigating performance problems at your backend.
How to find closed SharePoint Web Parts and add them back
If you have closed a SharePoint Web Part and want to restore it edit the page where you have closed the Web Part. Go to the insert tab a click Web Part. On the left hand side there is a group called closed Web Parts where you will find them. Select the Web Part you want to restore and click add.