You dog has taken something disagreeable or eaten too much.  Diarrhea in dogs is a normal thing and needs a little attention to treat it at home. You should give your dog bland food like boiled hamburger with white rice. For more details about the treatment, check this answer

 Dogs are known to suffer diarrhoea from time to time from a variety of causes including stomach or intestinal bacteria or viruses, intestinal parasites, and dietary sensitivities or indiscretions such as scavenging rubbish or other offensive materials. This should not be a cause for concern but if there are other symptoms like vomiting, then you should consult a vet immediately.