Seem to have suddenly lost feelings for my girlfriend?

My girlfriend is also my best friend. We are best buds as well and we talk about about everything together. Lately, my girlfriend has become more depressed due to an aculation of work and family stress. She has put on weight which has detracted from her physical attractiveness. She also has become more emotionally distant. I saw her over the holidays and it seemed like she didn't want me around. Her eyes looked dead. She was not made-up and was wearing oversized clothes which were not attractive at all. She told me she wasn't in the best of moods and told me that stress had taken a toll on her, but this had been happening for quite some time. I felt sad when I left her house that evening. Something strange happened. I felt myself shutting down to her emotionally. Lately, when I thought of her, I didn't feel attracted to her and I didn't feel like she was even my close friend anymore. These changes in my feelings were sudden and they shocked me. We were best friends before we got together and I just fell in love with her. Now when I think of her, I struggle to feel anything. Occasionally, a glimmer comes through but only for a moment and then my brain reacts negatively. What the hell happened? I can understand feelings gradually fading away, but not suddenly. How often do issues like these happen in relationships? I'm scared of losing my best friend and scared of not being able to be involved romantically if the attraction was extinguished. Is there any way to rekindle the fire or is it forever gone once it's gone? I'm really upset about this and have cried myself to sleep over this. What can I do?