There's a time limit for the go-back such as 30 days or less after Upgrading to Win10. Otherwise you will have to do a clean install of Win7, hopefully you have the disc/s that were prompted to create when first getting the Win7 computer. Some brands/OEMs will have a procedure to do a factory restore using files on the HDD/Hard Disk Drive but will have a program useful for creating 1 copy of the factory restore discs onto CD or DVD R discs [not RW]. Lacking either of those will require a purchase.
For recovery when the drive works but Windows doesn't, I use a free Linux Mint LiveDVD [there's others also free
www.distrowatch.org ] to access files and copy to another drive such as a USB Thumb drive of sufficient capacity or an External USB HDD.
For the privileges issue you can also do a Reset or clean install of Win10 but again, back up what you don't want to lose, that exists nowhere else as they could be lost.