So I uninstalled each new program I had installed recently one by one until I narrowed it down to which one I thought was causing my particular issue. According to my friend at Apple, he was 99% sure it was third party software. I was able to get back into my system, but I noticed if I shut down and tried to turn it back on, the problem still persisted. See if this boots you back into the system.
Type the following commands, pressing enter after each command: Give it a few minutes until it doesn't load anything else on the screen. With the computer shut down, turn it on and hold down Command + S to launch a command shell.
Here's what helped me solve the issue without having to reformat my drive or reinstall the OS. I was experiencing the same symptoms: upon starting my iMac, the progress bar would load underneath the Apple logo, and it would just sit there and never progress any further.
I texted a friend of mine that does software development for Apple and he was able to help me get past this problem.
This solution worked on my 2011-vintage 21-inch iMac (iMac 12,1) with Mac OS High Sierra. If you're able to get into Recovery Mode utilizing CMD-R, and you're not afraid to work in the Terminal program, THIS WORKS! If you follow these steps EXACTLY, a bad file named "mbr_cache" will be rebuilt and your Mac will reboot successfully (slowly the first time as the rebuild happens, then normal speed after that). I found a couple of postings that explain this surprisingly quick solution. YOU MAY NOT HAVE TO ERASE YOUR HARD DRIVE, or reinstall Mac OS. I tried a number of approaches, but was confident that a simple solution existed. I'm a very experienced Mac tech support consultant, yet I've been grappling with the "too many corpses" problem since yesterday.