Being unable to straighten isn't terribly unusual, but it would be helpful to know first if this is a post surgery problem, is this due to limping for an extended time, are you in PT, are you in pain, etc.? Can you give some additional information so that people can make more meaningful suggestions.
If it is simply stiffened up after a surgery due to disuse or tightened up due to limping for a while you can try lying with a rolled towel under your heels for five minutes at a time as a starter which will put some stretch behind your knee. Little by little you will need to gently stretch out your hamstring and that is one way to start. When you sit in a chair you can prop your feet on another chair of the same height in front of you. Your knee will be pulled down by its own weight. I want to warn you not to try this one for more than a minute or so at first since it can be very uncomfortable.
Hope this helps. With a little more information I am sure that many more people will post suggestions as well.