I upgraded to Mac Catalina and my Iphotos cannot be accessed. What can I do to get them back?

You don't say what version you upgraded from. If you mean you can no longer use iPhoto (not compatible with Catalina), you need to open the iPhoto library using the Photos app.

Photos will then convert the library so it can be opened in Photos.

As with any major change, you should take a backup of your iPhotos library first in case anything goes wrong with the update process.

To open the iPhoto library, hold down the option key while starting photos. It will probably be listed in the resulting pane. If it is not click "choose other" and navigate to where your iPhoto library is stored.