How to recover lost serial number? I purchased Student and Teacher edition but have long since lost the code. Need help!

I've had my laptop for about three years and purchased Adobe Photoshop around the same time. It was the Student and Teacher edition since it was the only one available at that time. As of last year my laptop stopped working due to problems with the motherboard so I sent it to have it fixed. It's fixed now and all of my other programs installed on my computer are still there and working properly. The memory on my computer is fine and has not been affected, however, Photoshop has stopped working. For a minute when I had my computer back from repairs, it was working normally. Then after a day it started asking for my serial number or to keep using my 30 day free trial. I have long since lost my serial number and do not want to repurchase the program but I need to use it. What do I do and how do I find my serial number?  Please help and thanks in advance!