at&t and its MVNO's would allow 60 days before deactivating the number. I imagine that's still the case, even for H2O.
If so, you should still be able to see your number with a balance of $0 if you have registered an account with them online and it's been less than 30 day past expiration.
If it's been just few days since your balance expired, try talking to a CS for the chance of having it restored (as a one-time courtesy) if you put some money in it right away. It's worked for me with other prepaid providers in the past.YMMV.
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