I've had an MRI before surgery and was told that the meniscus wasn't torn. But after surgery I had MRI and now it the meniscus root is displaced how?

MRI. Meniscusc tear can happen at any time. Wear and tear can cause meniscus to get injured (we are alway walking and bending our knee), surgery, joint injection may cause untentional injury. Mri interpretation is challenging if you have subtle changes. During surgery you may see pathology not seen on mri. You may injure other ligaments, cartilage, meniscus, bone after success surgery.
MRI is not absolute. Mri is like a picture of a blind date. You never know what they look like till you go on the date. Surgery is the date.