Check that the email is legitimate, as some scammers try to trick you (phishing).
Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support
Identify legitimate emails from the App Store or iTunes Store - Apple Support
Go to you account settings/payment information manually (i.e. without clicking links in the message). If there really is a billing issue, then it should be noted there as well.
Settings﹥[Your Name]﹥iTunes & App Store﹥[Your Apple ID]﹥View Apple ID﹥[Sign in]﹥Payment Information
Any issue would be more precise than just “problem”, like: insufficient funds, expired card, card denied, missing CVC code, or whatever the issue may be.