If your account balance hits 0, you will be unable to send additional messages. Your current account balance is visible on your Account Settings > Plan page.
If you're on a Pay and Go plan or need to add to your account balance between subscription cycles, you can add additional "Top Up" account credit with an individual purchases.
To add account credit:
1. Go to Account Settings > Plan
2. Click the red "Top Up" button
3. Select the the desired additional credit amount, and click "Purchase." Your card on file will be processed, and the credits will appear in generally under a minute.