Oh dear, we are sorry to hear that your Neopet isn't feeling very happy right now! The first thing you are going to want to do is visit the Quick Reference page here: (http://www.neopets.com/quickref.phtml) to make sure that your pet is full of food and is not sick as these things can make any pet feel under the weather.
If your Neopet is feeling healthy, it might just want some new toys to play with! You can play with toys in your inventory by clicking on them and selecting “Play with Pet” from the drop down menu. If that is still not doing the trick, a sure fire cure for the blues is a quick trip to Mystery Island to take a Tiki Tour or a spin on Roo Island’s Merry-Go-Round!
Good luck, and we hope your pet feels better soon!