Please check server settings. This sounds to me like a case of fat fingering server address or settings.
Yahoo Mail offers standard POP3 access for receiving emails incoming through your Yahoo mailbox, by using your favorite email client software. To setup your email client for working with your Yahoo account, you need to select the POP3 protocol and use the following mail server settings:
Yahoo Incoming Mail Server (POP3) - pop.mail.yahoo.com (SSL enabled, port 465)
Yahoo Outgoing Mail Server (SMTP) - smtp.mail.yahoo.com (SSL enabled, port 995)
POP Yahoo! Mail Plus email server settings
Yahoo Plus Incoming Mail Server (POP3) - plus.pop.mail.yahoo.com (SSL enabled, port 995)
Yahoo Plus Outgoing Mail Server (SMTP) - plus.smtp.mail.yahoo.com (SSL enabled, port 465, use authentication)
I have created a test account and was able to sync to my phone without issue.
Try entering server address again.
I would also perform a hard reset on phone by pulling battery.
Let us know if that works.