General:
* My tongue was tied. (Simple and common)
* My voice was choked. (Suggests a strong emotion like shock or fear)
* Words failed me. (Emphasizes the lack of ability to express oneself)
* My mind went blank. (Focuses on the cognitive aspect of being speechless)
* A lump formed in my throat. (Physical sensation that can accompany speechlessnes)
More specific:
* My heart was a drum, beating so loud I couldn't speak. (Suggests overwhelming emotion)
* My breath caught in my chest, like a bird trapped in a cage. (Suggests being trapped by emotion)
* I was frozen, a statue in the face of such beauty. (Emphasis on awe)
* My brain was a tangled mess, and my mouth couldn't untangle the words. (Suggests confusion or overwhelm)
* My eyes were wide with wonder, but my voice was lost in the silence. (Focuses on visual reaction)
More figurative:
* My thoughts were like whispers in a hurricane. (Suggests being overwhelmed by emotions)
* My vocabulary had vanished, leaving only a gaping void where words should be. (Emphasizes the complete lack of words)
* I was adrift in a sea of silence. (Suggests being isolated and alone)
The best metaphor will depend on the specific situation and the emotion you want to convey.