Ehhhh... ...
Did he turn gray? Did his portrait disappear from the bar on the right of the screen? If that is the case, poor Minsc got Petrified.
Buy a Scroll of 'Stone to Flesh' and use it on Minsc.
Other than that, try Remove Paralysis, a Cleric Spell or Dispel Magic.
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If not a Sorcerer, then a Paladin.
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Baldur's Gate Item Mart