First, identify what your planter box is made of, because the repair method depends on whether it is wood, concrete, plastic, or metal. If the crack is very large, structural, or the material is crumbling badly, reinforcing or replacing the planter may be safer than a simple patch.
Empty the planter if possible, remove loose soil and debris, and let the cracked area dry. Clean the crack with a stiff brush and remove any loose material, old sealant, rot, rust, or flaking paint. If the planter is old and painted, take care with sanding or scraping, since older coatings may contain lead.
For a concrete planter, widen hairline cracks slightly with a wire brush, then apply a concrete patch or masonry epoxy according to the product instructions. For deeper structural cracks, add wire mesh or use a stronger mortar repair. After it cures, seal the inside with a waterproof masonry sealer suitable for planters.
For a wooden planter, remove any soft or rotten wood, let it dry, and fill the crack with exterior-grade wood epoxy or a suitable wood filler. Reinforce weak corners or long cracks with metal brackets or screws, then seal the wood with an exterior waterproof finish.
For a plastic planter, clean and roughen the area around the crack, drill small stop holes at both ends of the crack to prevent it spreading, and use a plastic repair epoxy or patch. Finish with a flexible waterproof sealant.
For a metal planter, wire-brush away rust, apply a metal epoxy or patch, and coat the repaired area with rust-preventing paint.
Finally, check that drainage holes are clear, add a liner if the repair needs extra protection, and refill with fresh soil. Let all repairs cure fully before watering, and avoid overwatering, which can stress old planter boxes and make cracks worse.