My MacBook Pro has a circle with a line through it, what do I do?

Recently I had issues with my MacBook Pro laptop booting to the desk top. I've taken my laptop to the Apple Store for assistance, which was successful. Since that time, I have Time Machine back up on a regular bases. This time around I had issues with connecting to my wireless printer. After finding my printer, I rebooted to save the settings. Now, my laptop will not boot to desktop, and the display with black screen with a line in a circle. What can I do, other than making an appointment with Apple Tech?