Sometimes the final charge is higher than the amount first approved, for example with hotels, car rentals or transport. If that takes your balance more than $1 below zero, your account moves to Negative balance.
While it's negative:
Card payments and bank transfers are paused, and your card is temporarily blocked.
You can still deposit or buy crypto to top up.
To fix it, bring your balance back to zero or above. Topping up part of the shortfall isn't enough. Once it's covered, your account and card return to normal automatically.