Is there a way to bring back objects that has been deleted or disappeared?

I don't think so. You can do a reverse deployment tho from another environment if you have any. 

Disappeared mean, you remove it from application. If remove then you can get it back from the environment itself but if you deleted it then as Harshit mentioned you have to do the reverse deployment for those objects.

Agree with the others for most objects, your best bet is importing from another environment or from any saved export zip files.

However, here is one solution for recovering process models, not sure if this is doable in cloud but we have done so on prem:

- Get the PM UUID from application export logs, or by editing running instances, File -> Properties

- Search the server file structure for UUID under <APPIAN_HOME>\_admin\models 

- Download the latest XML inside the directory

- Open the process modeler -> File -> Import, select the XML

- Save & Publish