So, this is what I personally do. Open Blender like normal with the model inside, then just export it as an FBX (that way we can see the bones we're working with), be sure to select the path type as copy so your model will retain any weights or shape keys it has. Always make back ups.
Shove that FBX into a new Blender project. It SHOULD appear as bones now (the rig) and its gonna look VERY weird and very intense.
What you want to find is the MAIN bones, Pelvis, Spine, Neck, Head, Eyes, Arms, Legs, etc and be SURE they are properly parented to one another, not just drifting without a parent or accidentally parented to the wrong bone.
The whole process is really hard to explain.
I just made a video to help explain better, but I doubt it will.
I hope it helps.
Link: https://www.youtube.com/watch?v=pD5ypf3iMVY