the only time I've had that problem is when the network connection didn't connect! 8O
Seriously, though, using a wireless setup, if the dhcp server doesn't provide an IP address, the local IP address is set to a microsoft standard internal ip address (something like 169.x.x.x). You might check to see if things are working on your network.
There are a few utilities that will tell you your IP address if it changes. Check into those. One is
http://www.isimus.com/IPer/. More can be found at
http://www.cewindows.net/faqs/networkutils.htm.
If you are connecting via the ActiveSync -- make sure the ActiveSync is green!
Good luck,
Dan