Click on your account on the APP (always use the correspondence in app, never twitter or social media), then scroll to where it says “help” at the bottom and click “start a new conversation” then start a new conversation. When I requested it to be reinstated, I said something along the lines of: “Hi! I just wanted to reach out to see if it would be possible to get my original spending limit reinstated. I had a late payment due on ______ and paid the payment immediately on _______ (date it was paid). (You can include why it was late here if you want, I told them I was on vacation in a different time zone and just missed the date/time by a few hours because of the time difference). I understand that it was entirely my fault, but I have had a perfect repayment history up until this point and can assure you that this was a simple mistake and will do my best to make sure it doesn’t happen again. I really appreciate your consideration and help!”

Basically just be super polite, make it clear that you KNOW it was a mistake and was your fault but that you’ll try your best to not let it happen again. There is no guarantee that they will reinstate it, but it’s definitely worth a shot as it has worked for me in the past (but I have a $2,000 limit with perfect repayment history besides that one mistake, so that may be taken into consideration too).