Pontiac Grand Am 2003

I have a 2003 Ponitac Grand AM with 68,000 miles. The low coolant light came on about 3 weeks ago. I went to a local mechanic and he adviced that I put more coolant in the car because the coolant was low, which is what I did. The light has came on again. I put more coolant in the car. I placed the green coolant in the car not knowing there was a difference in the different types of coolant (orange and green). I went in to get an oil change and the mechanic told me that he could not change the oil because there was water in the oil. He also told me that there might be a blown head gasket or cracked gasket. What could the problem be? Please help! When I take the car to the mechanic what should I say. :(