Sometimes you may halt a job that you're working on -- perhaps intentionally, so you can work on something else; or perhaps accidentally, by hitting the wrong key. (For instance, I sometimes accidentally stop a text-editing job by pressing control-Z.)
If there is only one stopped job, you can resume it by typing the command "fg".
If there are several stopped jobs, you may need to specify by number which one you want to resume. You can get a list of your stopped jobs, with numbers, by typing "bg". To resume job number 3 (for instance), type "fg 3".