The second step in the process to determine theme is explaining why the events of the story happened. Therefore, you need to explain why the events occurred, not what the events were.
We know that the events are that the grasshopper was cold in winter, and asked for a few grains. We also know that the ants refused to help him. Why was this the case? The reason why these events happened is that the ants worked all summer in order to prepare themselves for winter. On the other hand, the grasshopper spent his time singing, and now has no grain to eat.
Therefore, the theme will look something like this: "It is important to plan for the future." Not doing this is what led the grasshopper to his situation.