Tongue-tied

Adjective

 * 1)  unable to speak; at a loss for words
 * If you have never practiced at interviewing for a job, you are likely to have a few tongue-tied moments during your first interview.

Thesaurus
anaudic, aphasic, aphonic, breathless, brief, brusque, close, close-tongued, closemouthed, concise, curt, dumb, dumbfounded, dumbstricken, dumbstruck, economical of words, inarticulate, indisposed to talk, laconic, mum, mute, quiet, short, silent, snug, sparing of words, speechless, stricken dumb, taciturn, terse, tight-lipped, tongueless, unloquacious, untalkative, voiceless, word-bound, wordless