Did you build this PC yourself, or is it an OEM (Dell, HP, Acer/Gateway, etc.)?
If you did build yourself, it would be helpful to know the Motherboard make/model. Your post is also confusing as you said you were originally running Win10, installed the upgrade, and got stuck. Then it turns out that Win7 as originally installed on that computer. If you can't get a hold of install Media as Kemical suggested, then you might need to take it to a repair shop and pay them to reset it to Win7 factory condition. They may have to order Recovery discs, but if it's a self-build, you'll have to probably purchase an actual Microsoft disc+license key in retail packaging from ebay or amazon. This will cost from $80-$140 or so. If you or the repair Tech can get the PC back to Win7, then you can reinstall the Win10 upgrade per the instructions Kemical gave you. You can also just buy the Microsoft boxed retail disc media which is available all over the Internet for $120, and attempt to go straight to Win10. It can be quite tricky to burn the ISO file onto media, and as I've posted several times, the DVD is the only install media option at this point, as the USB stick method does not appear to work at all.
If you have an OEM machine (Dell, HP, Acer/Gateway), this can be greatly simplified by using the factory Restore partition hidden on your hard drive. This can usually be accessed with a keyboard combo keystroke such as <Fn-key+F10> or <Fn-key+F12> when your PC is first booting. This will take you into the PC manufacturer's recovery menu (not Windows) where you can select factory Recovery or Reset options. This will reinstall the factory load of whatever OS was on your PC when you bought it; in your case Win7. From there, the instructions Kemical gave you will allow you to choose Win10 upgrade or Win10 Clean install from media (ISO file). This option may still work for you, especially given that you say you can still boot into BIOS on the machine. As long as your hard drive hasn't failed, there's a chance this will work for you.
Hopefully, along the line, you backed up any and all PERSONAL DATA TO EXTERNAL MEDIA from your original Win7 setup on that PC. If not, when performing the Factory Recovery or Reset option from the Hidden Partition will ERASE all information that you previously had on that hard drive (Documents, Photos, Music, Videos, Emails, etc.)!!!!
Let us know what you have and we'll advise you further if need be.
best of luck,
<<<BIGBEARJEDI>>>