The OP has just discovered why it's IMPERATIVE that one make a fully-bootable cloned backup of one's internal drive (using either CarbonCopyCloner or SuperDuper) BEFORE attempting a major system upgrade.
If the OP had such a clone, he would be back up-and-running now, with no need to post here.
But that ain't the case.
OP:
I believe you can re-download Sierra here:
http://osxapps.itunes.apple.com/app...-c51c-c3c1fececb99/jze1425880974225146329.pkg
WARNING: this initiates a download directly to your drive.
Assuming the URL above works for you, put the Sierra installer into your applications folder, BUT...
DON'T RUN IT YET.
Next, I
STRONGLY ADVISE that you create a bootable USB installer for Sierra by doing the following:
1. You'll need a USB flashdrive of 16gb or larger, initialized to HFS+
2. You'll need either Install Disk Creator or DiskMaker X. I used IDC and got it from here:
https://macdaddy.io/install-disk-creator/
3. BEFORE you run IDC, I suggest you make A SECOND COPY of the Sierra installer somewhere else (do you maintain a drive for archived software?). I believe IDC will delete the installer app from the Applications folder once the USB flashdrive is done.
4. Now, open IDC. Select your source file (the Sierra installer), and select your target (the USB flashdrive). Then... let 'er go.
5. When done, you should have a bootable installer on the USB flashdrive.
6. You can use this to boot the Mac and re-install Sierra.
HOWEVER -- I don't think Sierra can be installed "over" HiSierra, which means you have to re-initialize the internal drive first.
YOU DO HAVE BACKUPS OF YOUR DATA, DON'T YOU ?????
7. Once done, your Mac should boot with the fresh copy of Sierra, and your next step is to restore your data using setup assistant (or manually).
This should "get you back to where you once belonged".
But do you now understand why YOU SHOULD HAVE HAD a bootable cloned backup before you even started?