How to clean retainers after it's been accidentally dropped into the wash basin, floor etc?

Gently. Gently clean retainer with soft toothbrush or soap and water. Avoid toothpaste as it scratches and dulls the plastic. You can then sterilize it by using Denture Cleaner Tablets (follow package instructions) or by soaking in 1 part bleach/3 parts water for an hour, then rinsing thoroughly.