Type chrome://extensions in the address bar and scroll through the extensions list, making sure the toggle from the lower-right corner is set to active next to each of them.
To disable an extension, simply toggle it off.
Alternatively:
Visit the different pages for Prepd’s Chrome extensions in the Chrome Web Store, and make sure that you “enable” the apps (there is a small, yellow banner displayed at the top with the following message: “This item has been disabled in Chrome…” select “Enable this item”)