Traffic freezes when your subscription ends either because you cancelled it or if the automatic renewal cannot get through for different reasons. It is not lost - it is held until you have an active subscription again.
"But I bought extra traffic recently"
That traffic is frozen too, and it is still yours.
Traffic you buy with Add traffic runs on the same clock as your subscription. It added GB to your balance, not days to your plan - so when the subscription ended, everything on that clock stopped together.
That is also why buying more is not the way out. Add traffic is only available on a live subscription, so it is not offered to you right now.
Only an active subscription unfreezes traffic. Use one of the two routes below. Everything frozen comes back, including the traffic you topped up.
Route 1 - reactivate your existing subscription
Use this if your subscription lapsed or you cancelled recently.
Click Billing in the left navigation.
Open the Subscriptions tab.
Find your subscription and click Reactivate Subscription.
Complete the payment.
Your frozen traffic is added back as soon as the payment clears.
Route 2 - buy a new plan
Use this if there is nothing to reactivate, or you want a different size.
Click Pricing in the left navigation.
Choose a plan.
Complete the payment.
Your frozen traffic is added on top of the new plan's traffic.
Checking it worked
Open your Dashboard. The frozen traffic warning clears and your balance shows the combined total - the new plan plus everything that was frozen.