Slide presentations?

Hi Sheila, thank you for your post, welcome to the forum, PowerPoint will delete a slide master from a presentation if it is no longer being used.
If you create a new slide master and then apply it to all the slides PowerPoint will assume you no longer want the old one and will remove it.
This can be prevented by using the Preserve Master button
Any new slides masters that you create automatically get the Preserve Master option so they will not be deleted
The default slide and title masters do not have this option set by default so you may want to change it.
To preserve a master switch to the master view and click the Preserve Master button on the Slide Master View toolbar
A little pin icon will appear to indicate that the master will be preserved.
This button is a toggle and pressing this button for a master which already has the Preserve Master option will remove the option and the master could be deleted.
Regards Pete.