If you're on mobile (iOS / Android), try to uninstall / re-install the app with the latest version.
If you're using Skype for Windows 10 app on your computer:
- Go to Tools / IM Appearance
- Ensure that Show emoticons option is set and Save.
If it still doesn't work you should reset it:
- Start menu > Settings > Apps & Features
- Choose Skype from the list.
- Advanced option > select Reset.
Note: your application's data will be reset but not your personal data like contact and profile.