all of a sudden I have no sound

[Maureen L Thomas <silverorb@verizon.net>]

I was on the computer and stopped to eat dinner and the puter went into 
black screen as it always does if left unattended.  It would not wake up 
so I changed the batteries in my keyboard and it still would not wake 
up.  So I turned it off with the switch and then back on.  Once it was 
on it worked fine until I tried to play a you tube and found I had no 
sound at all.  I checked the sound icon on the top right of the screen 
and it was set where I always have it.  I went to the sound thing to 
check that I have sound and as I tested both speakers I heard nothing.  
The computer seemed to think that was fine with a smiley face.

I am on a Lenova all in one computer with Debian 9.  I have not needed 
any upgrades lately so that is not the problem as my system is kept up 
to date with all software.

Any help would be greatly appreciated.

Maureen