old iMac is stuck on white screen no logo

How old is Your Imac?

You don´t tell my not!

The white screen of death,is not the trouble.

Year and what Mac you have and the MacOS will helpe.a lot?

Time Machine,and the enclosure,can be an, option on your Mac Problem?

I can not see why the HD shut, be dead on your iMac?

Post some more information on your Mac,and i will try to helpe you?

The next step would be to try to boot into Recovery mode. Turn your machine off, then hold the Command key and R key while you turn it on. Don't release them until you see the Recovery menu (It should only take a minute or two). If it successfully boots into recovery mode, then there is hope for your computer getting up and running again.

https://support.apple.com/en-us/HT201314

If the computer is dead but you believe the HD is okay then you can remove the HD and put it in an enclosure. You may then be able to get your data from the HD by connecting the enclosure to another Mac. If the HD is dead and you never had a backup for it, then the answer is no you are out of luck. As a rule of thumb, you should never operate any computer without a working backup plan. At a bare minimum you should be using Time Machine.