Did you recently uninstall and reinstall the app? Did you sign-in with a new mobile device?
Your coins can disappear because the information of certain items such as spins, coins, levels, etc. are stored locally in the individual device. This means that certain items will disappear when you delete and reinstall the game or get a new mobile phone. Contact the support team BEFORE getting a new phone if you wish to transfer your items to the new phone!