how do you reset iphone 3gs--locked up on itunes displayed

Hold your on/off and home buttons at the same time until you see the Apple logo.

Now you need to plug your phone into your computer and start iTunes to restore your data.

I you are using Windows take a look at this link, http://support.apple.com/kb/TS1538

If you are using Mac take a look at this link, http://support.apple.com/kb/TS1591

Q - When you plug it in to the computer, does the phone vibrate or possibly even beep?

If not, then try another cable and/or USB port. Make sure the cable fits into the dock connector fully.

Q - Does the phone appear to be charging when plugged into the wall charger (also vibrates/beeps when first plugged in)?

If there is no indication of it charging you may be looking at a hardware issue, in which case you'd probably do well to grab the next appointment at an Apple store or call Apple Care.

Qs - Do you have the latest version of iTunes installed? Are you using a Mac or Windows-based computer?

I believe the Windows version of iTunes has drivers that can render an iOS device undetectable if off/corrupt/missing/not updated. See this article: http://support.apple.com/kb/TS1538?locale=en_US

Hopefully you will not need to reinstall iTunes (iTunes in Windows must be uninstalled in a particular order).

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Being that it is already showing the connect to iTunes image, there is a strong chance that there is nothing for the phone to boot into (as in no data to be recovered from the phone). Thus, if the phone appears to be charging with a wall charger (beeping/vibrating when plugged in) then you could possibly try what is called a "DFU" restore.

This is a last resort measure...

Make sure you're connected to the internet. Plug your phone into your computer and open iTunes. Hold down the sleep/wake button while at the same time pressing the home button on the phone. Keep doing this until the screen goes black (even if you see the red slider, keep holding). Once it has gone black, count to 8 then release the sleep/wake button at the top--but keep holding the home button. You may have to do this a couple of times to work on your timing. If you've done it correctly, a message will popup in iTunes indicating that the phone is in recovery mode. Also, the phone will not display the connect to iTunes image, but rather should be black. Release the home button and click OK then click on the restore button. The whole process should take maybe about 5-10 minutes. Once the restore is complete, the phone will restart then reactive through iTunes (need an active internet connection and SIM card).

If you get an error messages during this process or if it fails to restore the phone you should probably see the folks at Apple.

Good luck!