If someone else modded it for you there are a few possibilities ...
- There is no chip installed at all, but the one who modded your xbox did a softmod without telling you.
Reason : a chipped box should boot backed up games / auto-installer discs.
If you're 100% sure a chip was installed:
* The chip shifted out of place (if it's a solderless one), one of the wires came loose or the chip died.
* The chip is turned off. Look for switches somewhere on your xbox.
Better open up your xbox and check if there's a chip at all ... it needs to be fixed/replaced anyway.
If there's no chip .... contact your modder and tell him he ripped you off.
Cost is $50 installed for the latest Aladdin Live.