My i-mac will not do anything but flash question mark folder, can't do anything. Also, can we do a backup in this condition and how?

See ➡️ If your Mac starts up to a question mark - Apple Support

It may or may not be possible to recover data, depending on what happened here.

Backups best happen before data loss, theft, damage, corruptions, or other errrors.

That means the computer cannot find the startup volume. Please follow the steps in the link provided. If it still won't start then please take the computer to your local Apple Authorized Service Provider to be repaired. Are you telling us, you were not backing up the computer prior to it's failure, if so then you are likely going to learn a very painful lesson on my backing up every computer a minimum of 1x a day is critical.

At a very bare minimum you should be using Time Machine, Back up your Mac with Time Machine which will only require an External Hard Disk such as a OWC Mercury Elite Pro. It's too late now, assuming I am correct but once you get the computer running then please begin using Time Machine!!!!

OTFC wrote:

Hi, The folder with question mark stays on always and won't do anything else. It is not momentarily

You have an iMac that starts up to an error.

The error here is a question-mark-folder icon.

The question-mark-folder icon means there is no bootable system present.

The startup then ceases at the question-mark-folder icon.

Because there is nothing to start.

You cannot get the iMac to complete the startup.

Potential causes of this can range from a repairable corruption, to a catastrophic storage hardware failure with complete data loss.

Further complicating this, you have no backups.

To learn what to do next to try to get the Mac to start, click or tap on the following blue-text hyperlink ➡️ If your Mac starts up to a question mark - Apple Support