Your tampon should be in for no more than 6-8 hours, or you could get TSS (toxic shock) which can kill you. Try a hot bath to get it out, failing that get someone to take you to the hospital straight away. I'm not exaggerating when saying you could die. Read the instructions leaflet that comes with your tampons.
It will cause alot of problems!! There's something called toxic shock syndrome! You really should go to the dr ASAP and get that taken out!!!! if you start to get a fever and can't get the tampon out you should go to the ER!! It's serious!!