I'm having the exact same problem with my Xbox. With no modchip in, boots past the flubber screen and to the green Xbox logo...but I get no MS logo underneath it and the system hangs there. If I keep pressing the eject button *many* times, I will *sometimes* get a service screen that gives me error code 16. Sometimes I can't even get the service screen to come up.
Keep in mind: it's an unmodded v1.0 Xbox with the original virgin (never tampered with) 8GB hard drive. All original parts to this Xbox are intact and being used, all cables are secure, etc etc. When I last used it, it worked...now about 2 months later, it does not. Unfortunately for me, being a v1.0 Xbox - the warranty has definitely expired...lol
Can't figure this one out to save my ass...any reasonable input is welcome and appreciated!