i changed my operational system and my unity don’t starts and it’s saying ‘‘THERE WAS A PROBLEM VALIDATING THE LICENSE BECAUSE THE OPERATIONAL SYSTEM IDENTIFICATION SEEMS TO HAVE CHANGED’’
Have you tried reinstalling unity? There’s a 5.2.3 update just come out which may fix the issue!
I believe this this thread over on the forums may help you resolve this.
There is also some information in the Support FAQ on the forums: