Can Platonic friends become more?

I've had a great friendship with a woman I met about 3 years ago. When we met she was in a long-term relationship and I'm not the kind to break up a relationship for my own needs. There are too many people out there to be that way. Anyway, she is single and over her ex and I find myself thinking about her in "that" way. We are great friends but I can't stop thinking that we'd be a great couple as well. I don't want to lose her friendship. What now?