First of all, don't panic. Not seeing any ribbons can only mean that you went off track. Turn around and go back the same way you came until you find the marked trail once more.
It's almost impossible to be off trail for more than a minute if you pay attention to the placed markers.
If you trek back and can't find the original trail, stop and try to get the attention of the other participants by blowing your whistle with three equal blasts until you hear someone responding with two equal blasts.
If you can't hear any other participant, obtain your location by using the GPS apps on your mobile phone and send a sms to the Organisers Emergency Number (OEN) with your BIB and relevant information and wait for a reply and further instructions.
Keep track of the time you spent off track and some landmarks that might be helpful to determine exactly where you are.