When memberships are purchased on TubeAssist with a Credit Card, the credit card is actually not processed until that evening. In the case where your credit card is declined that evening, your membership on TubeAssist is automatically downgraded.
It doesn't necessarily mean that you don't have enough money on your credit card - some banks simply don't allow recurring purchases online and there are a variety of other reasons that can cause it to fail.
You will need to sign up again using a different credit card or your PayPal account: https://www.tubeassist.com/users/upgrade-membership