I have seen this question a few times on the Office 365 forums:
When I first saw the response, I was slightly confused because I had never seen that particular behavior before. I tried to recreate it, but I wasn’t able to. What I have no learned is that is because I click the save button and do it quite often in my process. Because of this, I had never run into the issue. During one of my sessions at #SPCTCDC I was trying to add a web part to the page and was amazed that it just disappeared. After the session I went back to look at what was happening and realized that I was running into the same issues that others were seeing. While I will make no claims in this blog post to tell you *why* this is happening, I will try to cover what is happening and how you can avoid it.
Recreating the Issue
To see this behavior, start with a wiki page. Add a web part to the page and immediately use the tool pane to edit the current view. When you save the change to the current view your web part will no longer be visible on the page.
Where did my Web Part Go?
So where did the web part go? Did it really just disappear? The answer to that is no, and here is the trick to find the web part. Open the url to the page and add ?contents=1 to the end of the url. Here is an example of the link for my site: http://team.contoso.com/SitePages/Home.aspx?contents=1 . When you do this a web part management page will load and will show you the web parts that are currently associated with the current page. You will notice that this page thinks that the announcements web part is open on the current page.
This means that if I went through the process of simply re-adding the web part then I would get another web part added to the page, this time though it would be called Announcements . Based on the forum posts I linked to, this is a common scenario that people have run into.
How do I get it back?
There is a trick you can use to get your web part back. Select the checkbox next to the web part and click on the option to close the web part. This will place the web part in the closed web parts gallery and you will be able to re-add the web part to the site.
Once you close the web part it will be added to the Closed Web Parts gallery. The closed web parts gallery can be accessed by adding a new web part to the page. You will see it at the bottom of the menu (if you have any closed web parts on the page).
You can select and add this web part back to the page and it will add the web part that was lost when you were editing the settings:
I have seen some cases where the web part won’t re-add to the site properly. In these instances I needed to delete the web part (use the ?contents=1 trick and delete instead of close) and then re-add it to the page altogether.
How can I make this not happen?
So far the best thing I have come across that keeps this from happening is to add the web parts to the page and save the page before editing any of the web part settings. The save command seems to cause the web part to get added properly to the page and then allows me to make any changes that I need for configuring the web part. If you have run into this issue in the past it would be a good idea for you to take a few minutes and look at the different web parts that are currently associated with your page. If you have a lot of web parts that marked as open but not displaying on the page it would be good to take a few minutes and delete them from the page. If I find the technical reasons for why this is happening then I will be sure to update the post.