Hold during startup
Shift (⇧) Start up in safe mode.
Option (⌥) Start up to Startup Manager, which allows you to choose other startup disks, if available.
If your Mac is using a firmware password, you're asked to enter the password first.
D Start up from the built-in Apple Hardware Test or Apple Diagnostics utility, depending on your Mac model. Or use Option-D to start up from this utility over the Internet.
N Start up from a compatible NetBoot server, if available. To use the default boot image on the NetBoot server, hold down Option-N instead.
iMac Pro doesn't support this startup key.
Command-R Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-
Option-Command-R to start up from macOS Recovery over the Internet. macOS Recovery installs different versions of macOS, depending on the key combination you use while starting up.
Option-Command (⌘)-P-R Reset NVRAM (or PRAM).
If your Mac is using a firmware password, this combination causes your Mac to start up from macOS Recovery instead. Turn off the firmware password from macOS Recovery, then restart and reset NVRAM.
Command-S Start up in single-user mode.
T Start up in target disk mode.
Command-V Start up in verbose mode.
Eject (⏏), F12, mouse button, or trackpad button Eject removable media, such as an optical disc.
The flashing question mark means it cannot detect the hard drive. Did you happen to get any liquid on it or sit it in a puddle of water?
You can try internet recovery or booting off of install media and going to the disk utility to check the status of the drive.