I accidentally dropped water on it, and it seemed fine, so I continued using it. But after 20-30 minutes a red led light appeared on the mouse, and it stopped working.
Is there anything I can do, or do I have to buy another one?
Do you have any kind of limited warranty for the product? You might be able to return it, depending on the policy of the place that you bought it from if it was a fairly recent purchase.
I'm not completely familiar with that particular model, but you can try a similar method that can sometimes work for cellphones:
The idea is that the rice will absorb the moisture contained in the device.
Here's a link that advises on some additional details that I might have left out. Although that linked article is referring specifically to cellphones, the basic premise is essentially the same and has saved one or two electronic devices of my own.