Set the StopTime to just prior to the simulation failure and then plot the results. You may see one or more of the quantities in your model growing exponentially towards +/- infinity. This would indicate that something in your model is unstable and you should recheck all of your reaction rates and rules for errors. A sign error in a reaction rate will often cause problems like this.
If that isn't the case, then please provide some more details about your model and the errors that you are seeing so that we can provide further assistance.
