What should I do now that I've realized there's no more alcohol left in the house?
What happened to all the alcohol?
I feel like my friend took it all and is celebrating with someone else.
That sucks since alcohol can get quite expensive. It's not your fault that your friend wants to have fun too much with others and your alcohol. Make them pay for it next time you see them.
I don't want to be out of party fun. They should bone up.
If you helped pay for the alcohol and your only issue is that they are not inviting you to their parties, then you have very valid issues which you should talk about them with. Let them know that you would like to share the alcohol next time.