How do I resolve moving lightning bolt symbol on my Mac screen?

Does the look like this?

If so, you inadvertently booted into Target Disk Mode, and D.I. Johnson's recommendation should restore regular function. Should it NOT resolve with a restart, please post back.

Press and hold the power button until the Mac shuts down. Then start the Mac again normally.

If the symbols keep moving or change, or if your screen flickers or shows other weird artifacts, it could be a graphics card or display issue, which might need professional repair.