Has AdBlock vanished every time you open Chrome? Visit chrome://extensions and see whether AdBlock is still there and has simply been disabled or has been uninstalled entirely. Either way, it could be an issue with malware, an extension that doesn't like AdBlock, a broken installation of AdBlock, or even Chrome itself.
Look for malware
If AdBlock is constantly being uninstalled or disabled when you open Chrome, it could mean your computer is infected with malware even if you have a Mac. Macs are vulnerable to malware just as Windows computers are, it's just more rare. And malware extensions often look for and uninstall other extensions that can keep them from doing their nefarious work on your computer.
The first thing you should do is scan your computer for malware. Follow the instructions in this Knowledge Base article: How to remove malware or adware from your computer
Look for jealous ad blockers or other extensions
If that doesn't help, look for ad blockers that don't want you to use other ad blockers. One that we know about is the Adblocker Project extension. This ad blocker actually uninstalls any others you have. It's no longer available in the Chrome Web Store for this reason. If you still have it, uninstall it.
If you don't have the Adblocker Project extension, look for a similar extension. First reinstall AdBlock. Then disable all other extensions except for AdBlock and restart Chrome a few times. If AdBlock is again uninstalled or disabled, it means one of your extensions doesn't want you to have AdBlock. Enable them one at a time, restarting Chrome after each one, until AdBlock is once again uninstalled or disabled. The last extension you enabled is the jealous one. You'll need to decide which extension you value more, that one or AdBlock.
Extensions can become corrupted if something goes wrong in Chrome. One indication of a corrupted extension is that its button in the Chrome toolbar changes to a jigsaw puzzle piece. Chrome disables corrupted extensions. You can visit chrome://extensions and try to repair a corrupted extension, but it's better just to reinstall it.
Chrome also disables extensions when they request new permissions you haven't agreed to allow. A Chrome update introduced something that claims AdBlock needs new permissions and disables AdBlock until you accept them. We don't know what causes this message to appear. AdBlock does not need new permissions. You are perfectly safe in accepting the permissions AdBlock requests and allowing Chrome to enable AdBlock.
Have you have accepted the permissions but you still see that notice and AdBlock is still disabled every time you start Chrome? Reinstalling AdBlock should fix it.
Here's how to uninstall and reinstall AdBlock.
Look for a problem with Chrome
If the above suggestions haven't fixed the problem, then the problem is most likely Chrome itself. Please follow the instructions in this Knowledge Base article: Basic Chrome troubleshooting