Hopefully you made a copy of the EEPROM or the HDDKey.
If your modchip went south then you need a LOCKED hdd (locked to your xbox).
Reason being, a HARDMOD can support both LOCKED and UNLOCKED hdd's, on the other hand, a SOFTMOD requires a LOCKED hdd. Chances are your hdd is already UNLOCKED and now that its not hardmodded, it wont work. In this case you would use your copy of the HDDKey OR EEPROM to lock your HDD to your xbox.
If you are getting a error code 13, or 16 then your dash is screwed up.
If you want it fixed you should state the error code you are getting.