Restore from a recent WhatsApp backup
Check Google Drive backup on Android
Check iCloud backup on iPhone
Reinstall WhatsApp and restore the backup during setup
Ask the other person to resend the chat or messages
Check exported chat files if you previously exported the conversation
Check local backups on Android if available
Use phone file manager to look for WhatsApp backup files
Restore a device backup if your phone backup includes WhatsApp data
Contact WhatsApp support for account-related issues, not deleted chat recovery