I have a 2012 Mac that time to time get stuck in the loading screen or get a infinite white screen.
Cuz of that i already formated it 3 times, so now I'll make a guide of how to format your Mac Book. For everyone that needs it and for me who everytime that need to format again need to re-search everything another time cuz already forgot how to do it.
Anyway, lets go to the how to do everything that Mac says for you to do, and some others.
To Apple, the circle with a line through it, the Mac starts up to an Apple logo progress bar, the blank screen and probably some others is solved by macOS Recovery (press power button for 10s until your mac turns off then turn on and hold cmd + R). Then you repair your disk with the Disk Utility, if it don't work reinstall the macOS.
The Mac starts up to a question mark also have the things said before as a seccond option, the first one is to Reset NVRAM.
I'm not saying to just restart your computer cuz you probably have done that already, so what I'm saying is to Reset NVRAM, SMC, try to start in Safe Mode and to Repair the Disk. Proabably in this order and seeing if anything change step by step or, if you have backups of everything (that I do have cuz after formatting it 3 times, if I didn't had backups of every important thing I would be crazy), you can just format it already and get your beautifull 2012 OS (or whatever year your Mac is) that you'll need to upgrade after the formatting.
Shutdown the Mac, turn it back on and press (for those who don't know, the alt is also called option) option + cmd + R + P simultaneously. Release it after 20 seconds or once you hear the startup sound or when the Apple logo appears, whatever comes first.
If you are as lucky as I am that won't solve your problem, so now you'll try to:
Shutdown the mac, press shift + ctrl + option + power button for 7 to 10 seconds. If nothing happens is cuz your computer is with the another shift, in my case was with the right shift but has cases that is with the left shift.
If your computer turns back on and don't has your problem solved, you're like me bro. Lets try to:
Shutdown, turn on, press shift after the sound or when turning on, I tried both in mine, but in my case I couldn't get of the loading screen anyway, so I tried the:
Shutdown, turn back on, when you listen to the sound press cmd + R untill you see the Utility menu. If you're with your fingers hurt from waiting is cuz this command didn't work (I don't know exactly why, maybe the same reason that you can't use your Mac right now and are here to repair it). Shut your mac down again and then when turning back and you listen the sound press option + cmd + R or shift + option + cmd + R (in my case the first one) to start the macOS Recovery over Internet.
Then after wait a LOT you'll click on the Disk Utility, Continue,on the left top of the Disk Utility window click on View > Show All Devices, select your disk from the side list (the fist internal disk), choose First Aid > Run, if the process was right click Done and restart your Mac, if you still can't go to the log-in screen try to reinstall your macOS. If something is wrong with the First Aid you probably got an error that the Disk Utility can't repair, when this happens Apple say to erase (format) your disk.
Still on the Disk Utility select the Macintosh HD or the first item listed in the Disk Utility sidebar, click on Erase. You'll get a pop-up where you'll put the name that you want and didn't change anything else. Then you'll click on Erase. If the operation was successful click on Done, if you got something wrong Apple say that you might need service and to contact Apple Support.
Now lets do the Reinstalation of the macOS.
Return to the macOS Recovery screen, choose Reinstall macOS click Continue. Next, follow the on-screen instructions. Now you have your Mac in his Factory Version, that probably is very old and unusable. So now starts the harderst or at least most longer part.
First, look if your year have compatibility with the most recent macOS, right now is the Monterey, that my Mac hasn't compatibility.
If you have done the past steps (of disk erasing) without restart your Mac or leave the macOS Recovery zone, maybe you where lucky and could reinstall your macOS to a more recent version than your Factory Version, if you've restarted your Mac or where unlucky you're on the oldest one and will need to do the hard job.
In my case I'll upgrade my OS until the macOS Catalina who is the latest compatible version for my macbook. You can see the compatibility list here.
Since I'm on Mountain Lion and I want to go to Catalina I'll probably need to jump to an intermediate version that is capable to get the catalina one, since Mountain Lion can't upgrade directy to Catalina. After the OS instalation you'll need to use Safari to download the version you want or the intermediate version. Then do it again until you get on the version you want. It will take some time so do it at night is a good idea.
Since I didn't leave the macOS Recovery i could install the macOS Catalina directly, but I'm not the lucky guy so it didn't work (now instead of blank white screen I have the circle with a line through it error) the solution, as said before, is the same. So I just keept reinstalling and reseting the disk until it worked.
Now you should have your mac running without any problem.