old imac updated to sequia and now will not run

If you want a helpful answer, you'll need to provide more information.

For a start, what happens when you try to boot it?

Does anything at all appear on the screen?

.. and any other useful diagnostics?

dreb123 wrote:

when it booted up went to screen with a file and question mark-- i tried to restart in recovery mode and i was able to download an os from the internet--- was running fine for a day or so i think it tried to update again last night and when i started it up question mark was back

There are any number of things that can cause this, but if it was really running fine for a day and then the ? returned on boot up, that could be a sign that the internal drive is failing (or failed).

I think the next step should be contact Apple service: Contact - Official Apple Support

You can find locations of Apple Stores and Apple Authorized Service Provides (AASP) online on the Apple web site. For an older Mac like yours, you might do well to find an AASP, as these are third party shops that have all the specialized Apple tools and expertise, but they have more leeway to work on older Macs.

By the way, Apple's standard approach in cases like this tends to be to completely erase and reinstall a fresh copy of the latest MacOS version that the computer can be run with. For that of course, you need a good backup from which to restore your files. You can also do this yourself if you are comfortable with the process (and Apple provides online instructions) but if not, you would be better off having an AASP do it for you.

How old is the old iMac. Here is a list

macOS Sequoia (version 15) is officially supported on the following Macs: 

• iMac (2019 and later)
• iMac Pro (2017 and later)
• Mac Mini (2018 and later)
• Mac Pro (2019 and later)
• MacBook Air (2020 and later)
• MacBook Pro (2018 and later)
• Mac Studio (2022 and later)