Resetting Android Phone from CyanogenMod to Original
You'll need to download official firmware.  You can find it by searching "nexus one stock 2.x rom", and XDA might be better than Google.  I'd also look up how to use the Android SDK to flash it, probably using flashboot through ADB.  I have a Galaxy S which uses another tool, so I can't help with more detail without looking that up myself ;)