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What version of Mac OS X are you using ?
I'm writing your answer. Will reply in a few minutes.
The endless spinning might be an indication of hard drive failure. If the hard drive itself is failing, your Mac will freeze, shows spinning wheel, won't load properly etc. This also happens if the system files are corrupted. We need to identify what the problem is.
Since you already tried Safe Mode, now we can run Disk Utility by booting into OS X Recovery. Restart your Mac and immediately hold down Command and R keys and this will boot your Mac to the Mac OS X Utilities screen. From there, click click Disk Utility.
In Disk Utility, select your hard disk at left column. Under First Aid tab, click Repair Permission. After that, click Verify Disk and if asked to repair, click Repair Disk.
Then restart and immediately hold down Command, Option, P and R keys for 5 seconds and let the Mac boot up. Is it working now ?
Please let me know if that didn't work, so we can continue troubleshooting.
Steve, I'm sorry I will not be able to call you now. But I will open this call offer to other experts in here. If they are available, they will call you.
Now, I will help you here. If holding down Command and R doesn't bring Mac OS X Utilities screen, then restart and immediately hold down Command, Option and R keys for 10 seconds and this will boot into Internet Recovery. In Internet Recovery, click on Utilities menu and top and select Disk Utility. Run Disk Utility from there.
If that didn't work, then boot into OS X Utilities screen again then click Reinstall Mac OS X and follow the onscreen instructions to reinstall the OS X. Reinstalling the OS X will not delete anything. It will replace the old system files with new one. Let me know how it goes.
Restart and hold the Shift key until you see the Apple Logo, then login and restart the normal way
Did you do that? Are you still getting the spinning wheel?
Ok Reboot into the Recovery HD by choosing the ‘Apple‘ menu > Restart, then hold down the ‘Command’ + R’ keys on the keyboard during the start up process. When you see the Apple Logo you can let go. When the machine boots you may have to choose your language then you will see the OS X Utilities screen. Select the Disk Utility and then select Continue. Then in Disk Utility select Macintosh HD in the left column (if there are two with that name or it is named something else choose the first indented item) select the First Aid tab if it is not already selected, then click on Verify Disk on the far right. If that says “OK” in green type then select Repair Disk Permissions.
If that is all ok then quit Disk Utility and select Reinstall Mac OS X.