Hi Maria
The same thing happened to me not so long ago: all of a sudden I stopped receiving mobile notifications.
I'm going to guess that as Airbnb goes through system upgrades or system maintenance, sometimes default settings get changed (this has happened to me on other occasions also)
To ensure that you get mobile messages, go to your "Account" settings and look for "Text Message Settings". This is where you can check the appropriate boxes to make sure you receive them.
Hope that helps!
Travis