What Causes Locking Up Of Knees With A History Of Triple Bypass Surgery?

I am currently in a Nursing Home for rehab. I had a triple bypass done in November 2014. I have not been able to walk since then except for a few steps sometimes. It feels like my knees lock and will not go back when I try and rise up from the wheelchair. Hope you can help in some way as to why my knees lock up.