It's hard to know for sure, without knowing more details and the processes that lead up to them disappearing. But some possibilities:
- they were deleted...in which case you are out of luck
- they were moved or reassigned to another user (are you the administrator? is anyone else that maybe reassigned?)
- they were owned and/or in a group that is no longer available to you.
So the first option would be worst case. The others two might just mean you have to track them down and get permissions.
You may want to check out one or more of the tools on this page to help you find them or to know more about what is on your site.
Tools · Esri/ago-admin-wiki Wiki · GitHub
Most are free tools. For example ArcGIS Online Assistant is a simple browser based tool that allows you to do a variety of things
and GeoJob https://marketplace.arcgis.com/listing.html?id=c34019b0623041608df4d06970a7a96a with some free tools.
Retrieving data ...