Overheating Impreza Outback Wagon

I have a 1999 Subaru Impreza Outback Sport Wagon with about 135,000 miles. I bought the car with 71,000 miles. This morning I drove about 10 miles and when I reached my destination I noticed smoke pouring out from under the hood. I let it cool with the hood open, then discovered the coolant reserve tank appeared to be empty, although the main chamber was full. I filled the reservoir with coolant, and restarted. No smoke, so I drove the car home - about 10 miles. No smoke, still some smell... It has been sitting for a few hours, and I'm wondering if I have an urgent situation on my hands, or if it is safe to continue driving. In reading through some related questions, I am wondering if I may have a blown head gasket? Might I have caused damage with a one time low coolant situation? Thank you.