2009 Malibu won’t start

My girlfriend has a 2009 Chevrolet Malibu, it
has an intermittent no start issue. A majority
of the time the car works fine, but there are
times when the car does not start. You can
turn the key and lights come on but the car
doesn’t turn over, click, or anything engine
wise. The theft light is on. She had a radio
installed a couple years ago and don’t
remember the problem before that, but I’m
not 100%. I have heard of a padlock issue
but don’t wanna tear into the car blindly.
Thank you for any help, I hope I can find
some answers here!