Same thing happened to my wife (Galaxy S3, mail account at our ISP, Atlantic Broadband, POP account, running the default mail app). No problem accessing newer messages via her iPad or desktop PC running Thunderbird (all set to leave messages on the POP server) but the S3 suddenly did not see any more new incoming mail.
I futzed around with it for quite a while with no success. Finally I just switched her to the app I use, Maildroid, and all has been well ever since. In the process I finally got her switched over to IMAP on all devices to simplify mail managent.