12 to 24 hours after canning a jar will reach room temperature, and you can determine if the seal has failed. If the seal has failed, then you should remove the lid and check the sealing surface for any small nicks. If needed, you can change the jar or add a new and properly fitting lid. The food can them be reprocessed within 24 hours using the same time to process as you did before.
You may also want to adjust the headspace in unsealed jars to 1 and 1/2 inches. You can also choose to freeze jars instead of reprocessing. Single unsealed jars can be stored in the refrigerator and eaten within several days.