What is the exact error message? Is it "iPronto is unable to connect..." or does it say "iPronto was unable to load the requested page! Please check that the URL you typed is correct and iPronto is connected to the internet."?
You can check by typing file:/pronto/data/inet/loaderror.html as browser URL line. There is another error display at file:/pronto/data/inet/lowmemory.htm

What is the "General" setting for "LCD power off when AC plugged in (min)"? Is it set to "On" (always on)?

Try to ping the iPronto from another PC to see if the network is still up, take a look the "Status" page or check the diagnostics page of your WLAN router to check if the iPronto is still connected.

Can you still load other pages?