To be tongue-tied
To be unable to say what you want, often through shyness, embarrassment or intense emotion
e.g. I went to a concert of my favourite musician. Afterwards he was signing autographs but when I met my all-time hero, I just couldn’t speak; I was tongue-tied. Frustratingly, afterwards I thought of everything I would like to have said.
The idiom comes from the medical condition -ankyloglossia- where a baby has an unusually short and/or thick piece of tissue connecting the underside of their tongue to the base of their mouth - making tongue movement difficult, but the idiom is not literal.