Catalina

Installing a new OS is taxing on a computer, it does lots of work on the HD. If the HD is in poor shape to begin with when doing a OS upgrade this can push it over the edge and kill it. If the computer will not startup at all the HD may have failed and may need to be replaced. Remember the computer is 5+ years old and a HD can fail at any time. Please contact AppleCare and if necessary take it to your local Apple Store or independent Apple Authorized Repair Center to be serviced.

Good luck!

I have upgraded my iMac 27 5k (late 2015), my iMac 27 5k (2017), my MacBook Pro (2018) and my wife's MacBook Air (2019) and they all worked fine but I have had a client who had his wife's iMac 27 5k (2015) install fine but not his iMac 27 5k (2014) which has downloaded and started the install of Catalina but froze at three different attempts at different parts of the black screen with the Apple logo (getting stuck twice at around 5% progress and once at 98% progress)

It seems like from reading other comments elsewhere that the 2014 iMac 27 5k seems to have issues, but not the other years. We have been able to recover to Mojave both failed attempts.

So take the computer to your local Apple Authorized Service Provider and have it professionally diagnosed.

Sounds like same thing here. I’m able to get machine to boot up by holding command r or maybe it was shift while restarting. This allows me to start up again, but in Mojave. Tried again and same thing. Hoping I can replicate.

Worked with new version that was prepared the week of Oct 22

Yes, I had the same problem on my 2013 mini, Core I5/2, 500Gb SSD. Some things that got me past this that may help you:

1. Recovery Mode: First power up (down first if it is on) and then hold down Command-R as soon as you hear the <bong!> at boot-up. The Recovery Menu should come up, which will include options to reinstall Catalina or restore from Time Machine Backup. I tried reinstalling on the SSD which kept crashing during install and powering off, followed by hanging during power on. So, after that, I tried reinstalling to an old USB hard drive I had. Slow, but it worked. Still, that was slo-o-ow compared to SSD.
2. Restore from Time Machine Backup to go back to Mojave: Selecting this option from the Recovery Menu will show you the dates and now OS version, which is great and allowed me to select the last backup under Mojave. Bonus that now my 32-bit apps (MS Office) now work again. Mojave was so great... think maybe I'll just stop here.
3. If you happen to have a different disk that you can boot from, simply hold down the Option key when you here the <bong!>. This will bring up a list of possible boot drives and allow you to select another drive or partition to boot from.

Honestly, I've been so disappointed in Catalina's install and poor regression testing (Mojave wasn't awesome either, but at least I could get it to work, eventually) that I will no longer install on my boot drive but always install first to a test drive, so that I can switch back if it's buggy.