Please check if you don't have Plain text mode checked in your Gmail message settings.
Just click on Compose
Three dots in the right bottom corner
Click on Plain text mode if it's checked
Your messages now should be with formatting again.
Your Out of Office messages could also be in plain text.
In this case, please check out our dedicated article
You may have black arrow instead of three dots if you use old Gmail user interface. Please click on this arrow and the same mode should appear.