How is your Paypal account funded – if by credit card, it could be that the card has expired and you need to update the details (although Paypal normally give advanced warning of this). Your other option is to add another funding method – if you are using direct debit.
The problem is that this takes time, since Paypal requires the funding account to be validated, which involves them depositing two small amounts. Once you are aware of the deposit amounts, you can validate the account/funding method. Even if you bank on-line, this can take up to a week. If you need to wait for your monthly statement, you’ve got a long wait.