Dear Friend,
It is very very difficult, harsh, cruel, distasteful, and brutal situation for you.
However, there is always silver lining in every difficult situation therefore do not take stress. Here is some suggestion:
First, you can find out your roll number from screening test answer sheet.(if you have saved that sheet). Second, when you downloaded your admit card for screening test, you might save admit card in laptop, desktop or mobile so you can search you personnel computer to search out your soft copy of your admit card.
By the way, frankly speaking, I also forgot my roll number and even my admit card was missing from my documents.As good luck would have it, I found my screening test sheet where my roll number was written on top right corner.