What software did you use for the backup? If it was Time Machine, you should be able to boot your machine to the hidden Recovery HD partition. However, and I suspect you do not have the file "Install macOS Sierra", this method will involve downloading and installing the "appropriate" Mac OS onto your machine. Here are the "choices" of what you will get:
https://support.apple.com/en-us/HT204904
It looks like if you want Sierra (but not High Sierra, and I'd recommend not installing High Sierra yet), you would press the Command and R keys, and that "should" get you Sierra. Be aware, though, that this process is slow. Once the installation completes, you'll be offered the opportunity to "migrate"/copy all your need "stuff" from your backup.
You could try another approach, though. Are you using another Mac to post here? If so, what exact model is it?
