Issue: Sometimes we face issue in Odoo where icons on top menus do not appear in Odoo and which make Odoo screen appear with the default icon Camera/default picture. 

Hint: If you get a similar issue please check by going into the menu items menu under the settings (Settings/Technical/User Interface/Menu Items) and search for the top menu where you do not see the icon and then go to form view and open and make sure Web Icon File and Web Icon Image fields are well set with its values if not that may cause the issue to describe here. 

If it is not set then you will have to trigger computation of Web Icon Image which is a computed field in Odoo so that it can come again with the Download option.