Please Help!! Cannot generate thumbnails!!!

Hi guys here is my website, all of sudden due to some of the plugins my this website suddenly stopped generating thumbnails. I dont know what script it uses. This theme was provided me via freelance designer. so right now i cannot contact him. Can anyone please let me know what can i do…

Thanks

http://www.citizencomputers.com.pk/1-laptop/acer/acer-ultrabook-m5/