Vendor denied tix to convention events that came with pre paid package.

I paid for a vendor package to a convention in Las Vegas in Sept 2010. The package included 2 VIP passes and to entrance to all of the weekends events. The event is this weekend. For the pass 2 months the people running the convention changed the package and the price without notifying anyone. I sent several emails to the director of the event asking if my package would remain the same as when I purchased it. I received no responses from anyone affiliated with the convention ever. This morning when I went to register I was being given only entrance into 1 event and my vendor booth. Again i asked to speak to the person in charge and was told that they were to busy.
Do I have grounds to sue for the recovery of my money since the product that I purchased was not provided. I travel a long way to attend this convention and spend a lot of money to make this trip happen. I have proof that they changed the vendor packages. I downloaded as a pdf file every vendor package as they changed it online.
No where does it state that they can freely change a package in they term of conditions.
Is there anything steps i can take to get back the money I paid since i was denied the product? or is there any steps i can take to do anything?

I'm from California but the convention is in Las Vegas

From your description it does sound as if you might have a reasonable complaint. Stop now and figure out how much it will cost in effort and expense to pursue this matter. Then figure out the value of the 2 VIP passes, et c. Finally figure out the value of what you did receive.
You should probably try writing to the convention organizer and asking for the return of some of your fee.