It is natural the initial spark will die down a little. However if it has gone out completely that is a different story. You say you can not imagine being with anyone else so ponder this for a while. Can you imagine being alone? Can you not imagine being with someone else coz you're comfortable with your boyfriend and just 'used' to him being around? Or is it because you love the way he treats you and love the relationship you have? No one here can guide you on how you are feeling you need to explore this further yourself. Maybe even taking some time apart so you can examine how you feel without him around. In my last long-term relationship I felt the spark disappear. I was no longer interested in sex and he just felt like my best friend or brother. I tried to make things work and get that spark back but after 6 months realised it was futile. You say this has been 'sudden' for you so maybe for now acknowledge the feelings you have, explore them and see how you feel in a few weeks or months time (with or without time apart). Try not to make yourself sick over it. You will soon know for sure what to do. Good luck.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!