You can only install the game X amount of times. That said, say you have a desktop, a laptop, and for sake of giving an example, we'll say you also have a computer at your parents' place. So 3 total computers.
As part of DRMs (ubisoft is big on those) you only get 3 activation with the game. If you've installed the game on your desktop, and on your laptop, then you've already taken up 2 (out of 3) activation. Keep in mind that when you uninstall the game, you get that activation back!
But say your desktop crashed, you've just lost 1 activation which means you only have 2 total activations left. It's why you always uninstall your programs as much as you can before reinstalling windows.
Could it be that you didn't "uninstall" that game before and lost activations...and now you don't have any left?