Tell the dentist, or if you cannot manage that. tell his/her receptionist/nurse , or get someone else to tell him/her.
Get someone to go with you.
Try to concentrate on controlling your breathing...remember that breathing OUT is how you get rid of the tension, not breathing in. So, normal breaths in then breathe out as slowly as you can.
If you're still having trouble, ask your GP to refer you to someone who can help. They should know who is available in your area. If you happen to be near a dental school, they may well have staff there whose main job is to help people overcome their fear of dentists. You are by no means alone.
Please don't cancel or don't show up. It will only make things worse for you. Even if you can only get through the door to the consulting room and explain that you are afraid, that will be a huge step.
Good luck.