Sims 2 no longer runs on my Mac?

My Sims 2 was running fine for a while on my iMac and now it wont start up. I have it installed in my Macintosh HD and everything. What happens is I'll click on the icon inside of my Hard Drive and the icon will bounce up and down a few time and then go away! It won't open up or even give me a message! Help please!