Depends on the tone of your comic!
You could leave it as is, you could put a "..." over the crowd, you could write "~COMPLETE SILENCE~" in the background of the panel like a sound-effect if you want to be joke-y about it, but uh, that's obviously a more silly solution.
Another thing I might do is give the nervous speaker two or three ellipses, instead of the crowd -- like this kinda thing:
What ellipses do is establish a beat of silence -- essentially, they make time pass. If you leave the panel completely blank, you'll get a moment of silence, but it'll be a quick moment. I feel like putting a "..." over the crowd can read a little goofy, kinda like the "COMPLETE SILENCE" idea (which might work for the tone you want)! But giving the speaker a series of ellipses makes the silent panel stretch out -- her waiting for several beats in a wordless panel makes the panel long and the silence awkward.