ITunes only plays one song at a time ... help please

All of my songs are checked, but iTunes only plays one song at a time. I can't figure out why. Any suggestions? It just started doing this.

You don't mean you want it to play multiple tracks at the same time, do you? Like where you would hit play and ALL the songs would start playing at the same time?
Or do you mean iTunes will only play one song then stop rather than going on to the next one?