Problem with Acrobat Pro Reinstalling

My Acrobat Pro XI has stopped working, and I believe it is because some part of the program are accidentally removed or sth like that. I get the following error message:

Well, I tried to reinstall it. The problem is that I do not remember my serial number. And I did not use any sort of clouds for buying the software at the time. I just got the serial number and that was it.

• Now when I want to reinstall the software, it asks me whether I want to install the trial or if I have the serial number. I really don't know what to do.