Another reason might be a faulty model you imported from some site.
To solve such issues, I follow this process:
1 - Delete half of your models, and try to render.
2 - If render is successful, the faulty model is one of the deleted models.
3 - If render is UNsuccessful, restore models with UNDO button, and delete the other half. Try to render.

By repeating these steps you can quickly narrow down which model is causing problems.