Restart Freezes before completing

Hi JO58SBC,

First, try to reinstall macOS on your Mac. This shouldn't delete any data if your hard disk is healthy, however, having a backup is important. Unfortunately, it currently seems to be too late to create a backup.

To startup into Internet Recovery Mode, shut down your Mac, then startup into Internet Recovery Mode by holding Option-Command-R at startup. Then, reinstall macOS: How to reinstall macOS - Apple Support. If you cannot startup into Internet Recovery Mode, try to startup using the Command-R key combination. If you have a wireless keyboard, you may need to use a wired keyboard for these steps to work.

From there, see if the process completes successfully and you can use your Mac. If it doesn't, locate the serial number on your iMac (it's on the bottom of the stand, Find the serial number of your Apple product - Apple Support), and enter it here: Check Your Service and Support Coverage - Apple Support. Then, let us know the exact model year. We will then know what options exist.

Cheers,

Jack

That error means you have some sort of network issue. It is best to connect the Mac directly to the router using an Ethernet cable for a faster more reliable connection which also bypasses any hard to troubleshoot WiFi issues. You can try power cycling the router making sure to wait at least five minutes for the router to finish booting. The issue could be with your router settings, or the issue may be with your ISP or local Internet (the latter items you have no control over).

If you have access to another compatible Mac, then you can try creating & using a bootable macOS USB installer using the instructions in this Apple article:

How to create a bootable installer for macOS - Apple Support

To know which Macs are compatible you will need to know the exact model of your iMac which you can find by entering the system serial number here:

Check Your Service and Support Coverage - Apple Support

Now you need to find another Mac which can also run an OS which your broken iMac can also run so you can download the appropriate macOS installer (the host Mac does not need to be currently running the OS you are downloading). Usually you only need to be concerned about the following operating systems: 10.11, 10.13, 10.15, 11.x since these are usually the last OS available for any series of Macs. You can use the following article to see which versions of macOS are compatible with various Apple hardware:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

You can create and use a bootable Knoppix USB stick to check the health of the hard drive with the added bonus of telling us if this iMac can boot into a full OS. If Disk Utility is when you had a spinning wheel, then that tells me the hard drive may be the issue (or possibly the file system on the drive).

Download the Knoppix v9.x DVD .iso file with "EN" in the name for "ENglish". Use the downloaded Knoppix .iso file as a source for Etcher (Mac/Windows/Linux) in order to create the bootable Knoppix USB stick. Option Boot the Knoppix USB stick and select the orange icon labeled "EFI". While Knoppix is booting the Mac may appear to be frozen on the Apple boot picker menu so make sure to give Knoppix lots of time to finish booting.

When Knoppix reaches the desktop click the "Start" menu icon on the lower left of the Taskbar and navigate the menus to "System Tools --> GSmartControl". Within the GSmartControl app double-click on the iMac's hard drive to access the drive's health report. Post the complete health report here. You can also use this app to run the short and long internal drive selftests as well if you like. There may also be a tab which may show the last five errors if they exist which may be useful (it has been a while since I used this app, so I may be mistaken about this).

Before you embark on the solutions already posted can I suggest (if not already done) removing all USB devices before doing a restart attempt.