I have had this happen a couple of times.  I cannot access my ISY-99 by entering its IP adress, through network (it does not appear in network), or through the web site http://www.universal-devices.com/supp.htm (My ISY). 
  
I unplugged the ISY and now it works fine. 
  
What's the deal?  Is that normal.  I am going out-of-town, and do not want it to lock up while I am gone.