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It sounds as if the installation only made it part way through, and restoring to windows 7 is not an option, if you have the option to restore and you are not using a CD, it is likely not going to work as that partition should have been removed before beginning installation (if clean install) and the Windows 10 upgrade does not create a restore point that would be applicable. There are very few options here, as the Windows 10 installation has already deleted/formatted the Hard Drive, and you would have to have a CD/DVD/Flash Drive with bootable media (Windows 10 CD/DVD, Windows 7 DVD, any bootable drive) and then you would need your original Windows 7 product key for reinstallation of Windows 7 if that route is to be taken (and it sounds like the best viable one). If this is a laptop your product key will likely be on a sticker on the bottom of the laptop or under the battery (you will need to remove it). If this is a desktop, it is unfortunate that the product key may already be lost.
If you have a Windows CD or DVD, you can perform a "clean install" to remove the Windows 10 pre-installation software.
Unfortunately options are limited for this issue.
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