I have an old Macbook Pro which I hadn’t used for a while (at least a few years), and it just wouldn’t start up correctly. i.e. it kept “hanging” on the main Apple loading screen (where you see the apple logo and the progress bar as it loads the operating system).
I tried a few things but nothing seemed to work, except for holding down
CMD+S after hearing the startup chime on powering up. Doing this takes you into a single user mode (basically, a raw terminal where you can type commands).
Launching the file system check tool (standard in any Unix/Linux distribution) with
fsck -fy revealed I had a disk issue (which it fixes for you). Then repeating the command afterwards showed everything was ok.
I rebooted my machine, and everything was back to normal! So starting up in single user mode and running a file system check is worth a shot if your system appears to be stuck on bootup – who knew eh! 😉