If a bill has vanished, or there is an unexplained gap in your bill numbers, there is one very likely cause.
Someone deleted the purchase order
Deleting a purchase order deletes every bill raised against it. There is no warning, and the bills cannot be recovered.
Check: can you still find the purchase order the bill belonged to? If the order is gone, the bill went with it.
What to do about it
The bills cannot be restored. To rebuild the record you need to recreate the purchase order and re-enter the bills against it, using your vendor's original invoices.
Before you do, work out who deleted it and why — if the order was deleted to remove one bad bill, that is worth correcting as a habit, because it takes the good bills with it.
How to avoid it
Always open the Bills tab before deleting a purchase order. If there are bills, do not delete — use Cancel Order instead. A cancelled order keeps its history and its bills.
Delete an order only when it was raised by mistake and has nothing attached to it.
If the order still exists
Then the bill was deleted on its own, which is a smaller problem — just re-enter it. See Create a bill.
Still stuck?
Gather this before contacting support:
the bill number, if you know it
the purchase order number it belonged to
roughly when it went missing