The recently announced balance patch for Hearthstone is now available. Following its installation, those playing on PC may be unable to open the game, but there's an easy fix.
After downloading the new update, those who are affected receive an error that states, "Unable to load mono library." If you're one of those people, there's no need to wait for another patch to resolve this. All you need to do is click the Options button in the upper-left corner of the Battle.net launcher (it's right above the main carousel, next to the Redeem a Code button) and select Scan and Repair. It's toward the bottom of the list.
Once you've done that, the launcher should take care of the rest, identifying any corrupted files and resolving any issues. After that, you should be back in business.
Of course, you may not want to be playing right now if you're a Yogg-Saron user or a Warlock player, as both of those were nerfed in today's 6.1.3. update. A number of Basic and Classic cards are tweaked as part of this patch, including Rockbiter Weapon, Abusive Sergeant, and Charge.
You can read more about the changes and why they were made here, or check out the specific card adjustments in the image gallery above.