You can start wearing it again if it still fits you, or if it is only slightly tight fitting.
If it fits on easily, just start wearing it at night, it it goes on, but it's a little tight, wear it 24/7 (except for eating/drinking/brushing) for 2 - 4 weeks, then start wearing it at night only. If it doesn't fit on your teeth and it won't seat all the way down on them, go to see your orthodontist. If you are not sure if it fits correctly, return to your orthodontist. He or she will be glad to help you with this.