In case you upgraded by mistake your system from Wheezy, to the latest unstable version Stretch, you have several options:
1. Reinstall (recommended)
This is actually the recommended option, since the init scripts have changed across versions. Also it's pretty hard to guarantee that the same packages will be there.
2. Downgrade Your System
This option is a bit trickier, fortunately Jules from inspire.me wrote a pretty good article about it. Note that the tutorial he wrote was to downgrade from Jessie to Wheezy. Since we want to skip Jessie as well, we need to use the oldstable (aka "wheezy" at the time of writing) instead stable. See https://www.debian.org/releases/ for the current Debian releases.
So since you want now to skip two versions, you need to have the file in /etc/apt/preferences with: