If you paid with an AMEX, you can use the below process to get started on the chargeback:

Step 1 – Log in with your existing Online Account details.
Step 2 – From your statement page select the transaction you want to raise.
Step 3 – Select "Have a question about this charge?"
Step 4 – Choose the type of inquiry you have and answer the following questions.
Step 5 – Once you have reached the Inquiry Complete page you will receive a confirmation screen.
Step 6 – Your raised dispute will then be seen in your disputes activity at the top of your billing statement. You will be able to cancel this at any time.

If you cannot do it online, then you can call customer services but they are a little busier than usual so you might have a small wait.

Hope this helps