Must one who stole in his youth and repented repay what he stole, even if he does not know its value, and will his prayers remain unanswered due to his sin?

You must return what you stole to the grocery store owners. If you do not know the exact amount, then estimate to the best of your ability until you are confident that you have returned the right, or more than the right, amount. It is not a condition to inform them; rather, it is sufficient to return it by any means. Sincere repentance erases sins and does not prevent prayers from being answered.