I cannot straighten my knee, I had knee surgery several years ago, ACL and meniscus. Could I have damaged more meniscus? Can I avoid another surgery?

Maybe not.... An acute inability to straighten the knee often results from a torn meniscus, or a "loose body" within the knee. In addition to a good examination, you would need xrays and perhaps an MRI scan. If this is longstanding, but you are functional and pain-free, perhaps an exercise regimen would be all you need...You need to see an orthopaedic knee specialist...