As suggested by others, it's a court order. Turkish officials demanded a few things from wikipedia. The main item was to remove all content where Turkey is shown supporting ISIS, but they didn't receive a response, and the repercussion was blocking access nationwide.
In case anyone doesn't know, Erdogan is enriching himself and his family by having Daesh/ISIS/Islamic state deliver oil to him at bargain prices and selling it, mostly within Turkey:
(and Google for it, there is a lot of proof of this)
So let's stop pretending that this is done due to justice or even legal "right". This is being done to hide the crimes of the people in power. In this case, yes, crimes, specifically the government selectively applying the laws, corruption, and the like.
It is not being done for justice, to protect anyone's reputation from falsehoods, or ... it's just being done to hide crimes by the government.
I wasn't defending what happened, was rather reporting their reasoning for the curious. In Turkey justice has become synonymous with Erdogan's and his party's interests unfortunately.
They prey on the less educated, and banning a major source of knowledge seems like a very smart move by them.
Turkey isn't the only implicit supporter of ISIS though. ISIS exists at the pleasure and to serve the aims of a whole host of "Western friendly" nation states.
As a bonus, their terrible doings remind Americans of the need to stay engaged in a region that by some strange coincidence produces a lot of the worlds gas and oil.
