Hello Wilson,
You can follow up the check number range thru transaction code FCHI and change the status if you loose the sequential number.
The check number is assigned to a Bill of Exchange document (Request when Customer and Payment when vendor). If you need to input the check and assign it to FI document you can use FCH5 transaction code to create the check assigning to the payment FI document. If you don´t have created the bill of exchange, you can do that by transaction code FBW1 - Request (Customer) and F-40 - Payment (Vendor).
I hope this helps you.
Best regards,
Gabriel Coleti