This isn't a "normal" problem, so we'll have to do some troubleshooting.

What version / edition of PingPlotter are you using?
I'm sure you've tried closing and restarting PingPlotter - does the "Trace Count" ever change from 0?
What packet type are you using?
What "Engine" settings are you using (timeout, time interval, etc.)
If you're using PingPlotter Pro, are you using a remote agent?
Have you changed software firewalls recently? Maybe you have a software firewall blocking PingPlotter's ability to access the network at all - make sure your software firewall is configured to allow PingPlotter to have access.

If you're using PingPlotter Pro, some of these questions can be answered by using the Help -> Email PingPlotter Support option. Even if you're using PingPlotter Standard, you can download the latest beta and use that feature in the new betas (http://www.pingplotter.com/news.html#NewsRelease1991)

- Pete