Drill down to the probable item (campaign or ad group) that you paused and check the small box to the left of it. Next find the Change status... dropdown and select the Enable option. There you go.
In addition, make sure that the campaign in which you paused the ad sometime back has not ended. In that case, you need to remove the end date from the campaign settings and then enable the ad to resume it as mentioned by Lakatos.
Just a thought!