Tongue-tied
Unable to speak easily because of nervousness or shyness.
Examples:
1- Mahatma Gandhi, who later became an influential leader in India's struggle for independence, was tongue-tied during his first public speech in London, overwhelmed by the audience.
2- Tom always practiced his marriage proposal in front of the mirror, but when the moment finally came, he was so tongue-tied that he accidentally asked, 'Will you marry... pizza?'