Widget Not Showing
Three things go wrong and they look different from each other, which is useful because the symptom tells you the cause. A plain blue link means Notion never made an embed. An empty block usually means the widget is gone or the account is over its limit. A broken image inside a working widget means the picture is missing, not the widget.
A blue link instead of a widget
Notion never made an embed. Delete the line, type /embed, and paste the link into the box it offers rather than straight onto the page.
An empty block
Usually the widget was deleted or the account is over its plan limit. Open the dashboard and check that the widget is still listed and live.
A broken image inside a working widget
The image is gone, not the widget. This happens when the picture was linked from somewhere else and that source removed it. Re-upload it in the dashboard and the widget recovers.
Nothing is wrong with the Notion page in any of these cases. The block is fine, the thing inside it needs attention.
If none of that fits, write from the contact page with the page URL. If the site you are embedding is not ours, when Notion will not embed a site is the likelier page.
When it is not Notion
The same three symptoms show up in Coda, WordPress, Google Sites, xTiles and Craft, and the causes are identical. A plain link means the host never made an embed. A blank frame means the widget is gone or over its limit. A broken image means the picture is missing rather than the widget.
The one extra cause outside Notion is a content security policy on the host, which some WordPress security plugins add without announcing it.