Start without extensions

Hello,

Here is the Apple page describing all the key press possibilities when starting up a Mac. No combination systematically and permanently forces a startup without extensions, so the issue of a stuck Shift key was plausible.
Now, you can try this:
Resetting the PRAM:
Restart your Mac and hold down the 4 keys ALT-CMD-P-R and especially keep them pressed until the 5th 'chime' (sound at startup). Then release the 4 keys and let your Mac finally boot up normally.

then

Repairing Permissions
This takes place in "Applications / Utilities / Disk Utility ". Select the internal hard drive of the Mac in the left column, then choose the SOS tab, then click on the "Repair Disk Permissions" button and wait.

then

Repairing System Files
To do this, restart the Mac in "single mode" by holding down the Apple and S keys (or Cmd and S, depending on the keyboard) until a black screen with white text appears.
When nothing is moving on the screen, press Return (also called "Line Feed"): the new line that appears should now start with :/ root#
Now type the command fsck -yf and then press Return to execute the command.
- note1: respect case and spaces
- note2: in single mode, the keyboard is qwerty so to make the dash character (the one before the y), it's advisable to use the one from the numeric keypad.

Wait for the result of this command (which may take a few minutes).
As long as it contains “File was modified” then repeat the command fsck –yf and press Return.

Type reboot and press Return
The Mac will restart normally

and finally, if that still hasn't worked, you can Apple Laptops: reset the System Management Controller.

Ritchi