And just to rule out the most likely causes, even though it probably isn't any of these:
Do you have the Type tool?
Is the text that you want to change highlighted?
Are you on the right layer, and is it a Type layer?
If you click with the Type tool and then start to type, does it come in with the typeface you selected?
You are sure that you have the font that used in the file?!
If you haven't the font installed on you computer when you go to edit your text it will revert to Myriad pro Automatically.
In photoshop you should have the font used in your design to edit it in this font. If not try to use another font.
I recently had the same problem. In my case the solution was quite simple, once I found out the problem: some of the characters used in the Text, were not included in the fonts I choosed, hence could not be displayed correctly. Instead of just not displaying these characters missing in the font, PS simply decided not to display the whole text and jumped back to the last working font, that had these characters in the set.
Is this the same like your problem?
In your machine there can be fonts synchronized with Typekit, purchased fonts and fonts downloaded for free ... for fonts downloaded for free (very beautiful and that I have also installed) you have to rely on secure and reliable typeface websites,
You should not install corrupt fonts that tilt the system and you should not have, for example, the same font installed manually and synchronized with Typekit at the same time.
From the Creative Cloud app, you try to uninstall and reinstall Ps ... see if the problem resolves.
Another way to choose the type of font and a simple click on the font name at the top and you see the name highlighted in blue ... down arrow or up arrow to change type ... press enter to select.