no font could be found for family "Arial"

I was re-running some code that I hadn't run in a couple of months to make
barplots in R.  I didn't change a single thing in the script, but the plots
wouldn't work this time around.  The plot itself (the bars and axes) will
graph in the window, but no text appears.  In the console it says I have a
number of errors, all of which say "no font could be found for family
'Arial'".  
I have not knowingly changed anything in R and I would like to be able to
make barplots with labels and titles again.  Does anyone know how to fix
this?

Thank you!