The absolute simplest thing your Mac can do is to invoke Startup Manager. This code ia all in ROM, so the drive need not be happy or even present.
Hold Option key at Startup. Your Mac should chime, then draw a gray screen that is blank, except for a 'live' wait cursor, and over the next several minutes will add an icon for each discovered bootable drive, if any are found. When it thinks it has examined everything, you get the normal mouse-pointer back again.
if it can't do that it is so broken it will need service.
For the foot-tall dark desktop Mac Pro 2013 model (not MacBook Pro) this symptom can also be caused if your Mac has been off power, and its PRAM backup battery is also dead.
The long life Lithium coin cell is deep inside, and the process to replace it takes a 32-step near-complete dis-assembly. The procedure is complicated, but not tricky. One small side-effect is that this refreshes the wiping contacts on all the major circuit boards, removing accumulated corrosion and extending the unit's life.
Readily available CR2032 is a good replacement battery.