What's wrong with my modded minecraft?

Are the mods compatible with each other? Also are you using the correct version of mods for the version of minecraft you're using. I.e. Buildcraft 1.6.4 only works on minecraft 1.6.4. You can check what mods are compatible with what on the mods website.

I suggest that you restart your computer. If that won't work unload the mods and then load them back on! Xx

They're probably not compatible with each other or you just have too many mods. 30+ mods (depending on which mods they are) will make your game laggy and probably crash it. Otherwise, you might've downloaded the wrong versions (i.e you could've downloaded a mod made for 1.5 instead of 1.6.4)