There will be times where I know clearly what i want to say and can recite it in my mind. However once i try to physically say it I cant get any noise or air our of my mouth. What is happening?

For example, lets say i need to tell somebody "Billy went to walmart". I know clearly what i want to and i can clearly say it in my head. However once i go up to the person to actually say "Billy went to walmart" i cant get any noise physically out. I can still say it clearly in my head, but i cant get the physical sounds out of my mouth. What is happening? It feels like i cant even get air out of my mouth when it is happening. Im not trembling over one sound or word over and over again like a stutter, its a matter of i cant get the word out in general.