99% of the time it will be fine. It is likely some vandal has ripped it off to impress their mate(s). RM usually seal them up if they are not in service, though there is one in my area not in use, and a customer, who objected to it being removed ripped off the sealing plate and continued to use the box, until their mail stopped going anywhere.
Ring customer services and report it, or visit your local Collection Hub/Delivery Office and speak to them. Alternatively, tell your local Post Office and ask them to inform the collection hub, which they should have a number for, or they could mention it to their collection driver, though the driver who empties the box should have already reported the fact the time plate is missing to their boss already, and a replacement should be on it's way. We are required to display a time plate on all boxes under USO for customer info.