I would swap both as a precaution. Bad Y-main can possibly kill X-main and vice versa.
To repair: Just unscrew boards, unclip connectors, remove boards, install new boards, clip connectors, screw in boards, power up set (This may be done without the cover installed as long as you do not touch any internal part of the set in operation. A loud buzzing sound from the panel may be evident on bright scenes until back cover fully installed, this does not indicate a fault.)
After you see it works, go to the Samsung.com website and download all firmware updates for your TV to prevent repeat failure. This is a known issue with 2011 Samsung plasmas.