」「

'What's up, you're fidgeting.' 'Ah, well, it seems like the elastic in my underwear snapped.'

Literal

'What's-wrong, fidgeting' 'Ah-well, somehow underwear [possessive-の] elastic broke-[seems-みたいで]'

もじもじして describes fidgeting nervously or squirming with embarrassment — a 擬態語 (state mimetic). パンツのゴム 'underwear elastic' — パンツ means 'underwear' in Japanese (not 'pants'), and ゴム 'rubber/elastic' from Dutch/Portuguese. 切れちゃったみたいで uses みたい (inference) + で (causal te-form): 'it seems like... (that's why).'