Strip the trailer down to its frame to inspect the damage. Remove deck boards, sides and interior rails or rear gates. Get it down to the basic rectangle and yoke that fastens to the tow vehicle.
Assess the damage. If it has a broken spring or spring bracket, replace them. Put on a new spring or remove the broken bracket and weld in a new one. If a frame member is seriously bent, take it to a vehicle repair shop that straightens frames and see if it can be straightened. This may be possible even if two sides or elements are bent.
Cut out damaged sections if the frame cannot be straightened. Use an angle grinder to cut through the frame member. Cut back on both sides of the damage to solid metal. Cut and weld in a replacement frame section of the same size metal tubing. Reinforce by cutting strips of sheet metal and welding them to overlap where old and new sections join.
Replace the decking boards and sides with screws or cut new boards and sides if the old ones are damaged or deteriorating.