Phone Update not complete iPhone 3G

When I tried to update my phone yesterday, the computer said there was an error and it could not be done at this time. I then unplugged my phone from the computer because the only thing on my phone screen was saying I needed to connect to ITunes. The problem is when I connect to ITunes, it does not show my phone in the left side as an option. I can not turn my phone on or off or reset it and it just continues to show the connect to itunes picture....it has been like this for almost 24 hours....HELP
iPhone OS 3.1.2