Will Upwork tell you the reason why that contract is on hold?
That has to do with the client, and Upwork is not going to tell you anything about that client's private information.
Can I tell you the reason:
I can tell you that generally speaking this just means that the client's credit card information needs to be updated. Maybe the expiration date expired.
Is this something for you to worry about?
Is there something you can do about this?
What should you do now?
Do not work on the project until the contract is no longer on hold.