You are currently in debt. You owe the credit card people money, right now.
Yes, you will have to pay fees and interest. A credit card is not free money. You have to pay it back, with interest and any fees that you agreed to, even if it wasn't maxed out.
Depending upon your credit card agreement, your card could be declined if you attempt to use it. You could be allowed to charge additional amounts to it, but be charged an over-limit fee. The card company could also increase your interest rate up to the default rate (usually around 35-40%) because you have reached or exceeded your credit limit.
Your credit score will also go down. A big chunk of your credit score is credit utilization, or how much of your available credit you have used. If you have maxed out your credit cards, then you have utilized all of your available credit - this is interpreted as a bad thing by the credit rating agencies.