Have you started physical therapy? Does your surgeon want you to do straight leg raises? If you have the okay but haven't started PT, here's a trick. Lying on a flat surface (like a bed), bend your good knee so your foot is flat on the bed. Keeping your injured leg flat on the bed, flex your ankle so your toes are pointed at the ceiling and try to press the back of your knee into the bed. Have someone slowly lift your straight leg until it is even with your bent knee, then slowly lower it back down again. Do three sets of ten just like if you were lifting it yourself. This "wakes up" the muscles and reminds them what they are supposed to be doing! Eventually your muscles will start to help and you'll be able to do it yourself.
I've needed this kind of help after several of my surgeries. After my first surgery to repair a ruptured quadriceps tendon, the PT had to do this (and we did it at home) for three weeks before I could do a straight leg raise by myself. She told me it would click in one day, and she was right. One day I couldn't lift at all by myself, and the next day I could do all three sets of ten by myself!