Why can't I access the pouch?
This happens if the Cursed Medal is in your adventure pouch. When in your pouch, the Cursed Medal increases the frequency of hearts and rupees, but it doesn't allow you to open, or equip items from, your adventure pouch.
The Cursed Medal gets in your pouch unexpectedly if you have an empty spot in your pouch when you receive the Medal from Batreaux. I had this happen to me.
To get it out of your pouch, you'll need to drop it at the item check, which is in the Skyloft Bazaar.