Apple logo and progress bar appears but computer won’t start

Hi mhreeder,

I suggest you to follow this steps:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off. (Every Mac has a power button. On notebook computers that have Touch ID, press and hold Touch ID.)
2. Turn your Mac back on. If the issue persists, press and hold the power button until your Mac turns off. Then unplug all accessories from your Mac, including printers, drives, USB hubs, and other nonessential devices. You could have an issue with one or more of those devices or their cables. 
3. Turn your Mac back on. If the issue persists, once again press and hold the power button until your Mac turns off. Then follow the steps in this article: How to repair a Mac disk with Disk Utility. It explains how to start up from macOS Recovery so that you can use Disk Utility.

• • If Disk Utility found no errors, reinstall macOS.
  • If Disk Utility found errors and repaired them, restart your Mac. If the issue returns after restarting, reinstall macOS.

Let me know if this solves the problem for you!

p.s. unfortunately recently it happened to my old iMac too, I had to change the HD with a newer one