When working suddenly get black screen support/mac/startup

Probably a hardware problem, as a guess. Could be a corruption.

Hopefully you have a backup.

As for what to do, support.apple.com/mac/startup will take you to the following info:

If your Mac doesn't start up all the way - Apple Support

A prohibitory symbol, which looks like a circle with a line or slash through it, means that your startup disk contains a Mac operating system, but it's not a version or build of macOS that your Mac can use. 

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Turn on your Mac and immediately press and hold both Command (⌘) and R to start up from macOS Recovery. 
3. While in macOS Recovery, use Disk Utility to repair your startup disk.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.
5. If you still need help, please contact Apple Support.

Based on the above info, I’d guess that your storage is failing, or something has clobbered your install.

I already followed al the steps form the apple support.

Even with a new clean install the iMac will startup works for a couple of hours and than it will happen again.

Do you think maybe my harddrive is corrupt?

If you can get into macOS, run DriveDx (How to use DriveDx - Apple Community) and EtreCheck:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

Reboot29 wrote:

Do you think maybe my harddrive is corrupt?

I suspect your hard disk drive (HDD) is failing. One of the usual paths of HDD failure patterns starts with slowness and occasional beachballs, progresses to more general slowness, then to corruptions and crashes, then to complete failure.

If you do not have current and complete backups, that’s your biggest priority here. Not diagnostics, not testing, not re-installs, but to use whatever remaining I/O access is left here to fetch what may be the final available copy of this data…

Usual swap here is for an external SSD, if the HDD is failing. Thunderbolt 2 connection preferably, or the rather slower and correspondingly cheaper USB 3.x SSD. (Not USB 2.0. Too slow.) This if you’re not already looking at a replacement. If so… Here’s a recent iMac 24” M1 thread with some considerations: iMac ports - Apple Community

Your Fusion Drive (HDD + SSD) appear to be in good shape. Other than the fact you have no Time Machine backup, I see nothing wrong.

At this point, I would take the iMac to the Genius Bar at an Apple Store to see what they say: Apple Store - Find a Store - Apple and Get Support.

Cheers,

Jack

I'm assuming that the EtreCheck report was generated from the volume that was showing the corruption. This based on the device names shown.

Nothing obvious that might cause the corruption.

I'd still suspect the HDD/Fusion setup particularly if this persists, as SMART can miss a whole lot. Maybe related thermals or cooling, as that SSD is a little warm, but unclear what the trend there is. Or... a power glitch or some software corruption caused this.

Only other thing from all that is an opportunity for a memory upgrade, and for adding backups of any important data.