I'm afraid given the information you provide this is a little like fishing in the dark without a lamp. Like, what Magento version are you using?
That said, since Magento 1.5 that method is implemented on the catalog/product_view
helper, (as opposed to the catalog/product
helper referenced in the error message you posted).
I believe you must have somehow hacked the core code, messing up the Mage::helper()
factory method call in the Mage/Catalog/controllers/ProductController.php controller around line 67 to produce that error.
So instead of
Mage::helper('catalog/product')->initProductLayout($product, $this);
you want
Mage::helper('catalog/product_view')->initProductLayout($product, $this);