Sounds like this particular young Wizard has not equipped his spell deck!
To equip items, select them in your backpack & press the hand icon to equip the item. Items that have the little hand icon are what you are currently wearing/using.
Once the deck is equipped, make sure there are cards in it to cast! To put a card inside your deck, simply click on it on the right side of the screen and it will show up in your deck contents. The red circles next to the card shows the max number of copies you can put inside the current deck. The yellow circles show the number of that card that you have already put into your deck.
Also wanted to add that if you're switching decks, and plan on selling the other, to always remove any gold cards you may have in the sideboard. Then just re-add them to your current deck as you did in the other one.
Because otherwise when you get rid of the old deck, those gold cards will remain with it, and go bye-bye instead.
I learned this the hard way. Although luckily the cards lost were cards won in fights, not bought at the Library or Bazaar. Still, it was a lesson welled learned.
:( hi. my friend lost all but his weakest spell cards. can someone help please? 
