I can usually get this bottom C with few problems...but today it just sounds like a D....I'll play D and then press Ckey and the sound doesn't change.

Is there something wrong with the flute?

Are you sure that your embouchure has changed to get a sound out? Maybe one of the keys isn't going down.

Sounds like your bottom keys aren't closing.Check the springs.

check if the footjoint pads are closing 100%, even the slightest leak can prevent a whole note, ive had a similar problem with my G# key and wouldnt play from G downwards in the lower and middle octave. you should be able to get it sorted for free by a local woodwind specialist

your local woodwind specialists must be very generous...