1997 Volvo S90 Engine Lock HELP?

200,000 mi., 6cyl., 2.9L., oil fine, has been running fine with some coolant use. Last week only at morning start up had a valve rattle that would last a few minutes then go away with the rest of the day ok. Two mornings ago it just made a hum noise and won't turn. I have pulled the plugs, placed in neutral and can not turn engine by hand. I am in the process of pulling the valve cover and on to the head because I seem to have a lot of oil/coolant on the back of engine running on the Tran. May be something. I'm a little stuck because right now because I have pulled all the cover bolts and have tried everything I can think of to remove the star T40 bolts that hold the camshaft sensor on and can't get them to budge. Very little room to work (back of engine next to fire wall). I need help on how you would proceed from here and what to look for. I have replaced a head gasket and had the head machined on a Accord before so I think I can do this. This is my family's main car and I really need to fix with very limited funds. Please Help......