Shyness

My friend invited me to her party but i got shy because i dont know anyone. . But i still said hello and just sit alone lol. Honestly im so shy around people i dont know very well and haven't talked to in years. Well , i just cant seem to find anything to talk about with people noting come to mind. So i just wait for them to start the conversation and im also shyer around people. But i dont understand. . Why is this ? Do you get shy with people you dont know ?