That's a common and frustrating problem! Plastic bins are porous and can really trap smells. Don't worry, with a few simple steps using items you probably already have, you can get it fresh again.
First, give it a thorough clean. Empty the bin completely. Use a hose or shower head to rinse out loose debris. Then, create a powerful cleaning solution. A classic is mixing equal parts white vinegar and water. Vinegar is a natural deodorizer that breaks down odor-causing bacteria. For really tough smells, add a good squeeze of dish soap. Scrub every surface, especially the bottom and lid, with a long-handled brush. Rinse thoroughly.
For lingering odors, baking soda is your best friend. After cleaning and while the bin is still damp, sprinkle a generous amount of baking soda all over the interior. Let it sit for several hours or overnight. The baking soda will absorb the remaining odors. Then, rinse it out.
The ultimate natural deodorizer is sunlight. After washing, take the clean bin outside on a sunny day. Sunlight has natural bleaching and disinfecting properties that help kill odor-causing microbes and help dissipate any last smells. Leave it for a few hours if you can.
To prevent future smells, keep the bin dry. Let it air dry completely before putting in a new bag. You can also place a thin layer of baking soda at the bottom of the dry bin before inserting the bag, or use activated charcoal bags as a long-term odor absorber. Regular cleaning, about once a month, will stop smells from building up again.