Monitor Samsung SyncMaster SA350 stays black

I've been using a Samsung SyncMaster SA350 with my 2012 MacBook Air 11' for two years now. I use the Samsung as main monitor with laptop as secondary.

But since last Yosemite update, the monitor stays black. I tried several things : unplugging, change of adapter, change of Macbook Air (I tried with a 13' with the same update), but it is not working.

Samsung called me and suggested that I had a problem with my drivers and I had to check. But I have no clue how to do that ?

Any suggestion ? Thanks.