Sounds like you updated the dash with a Microsoft update,.....if thats the case, then you can no longer run unsigned code,...therefore your xbox is no longer a jtaged unit.
Hopefully whoever installed the jtag was good enough to take the precautions to prevent the e-fuses from blowing.
If thats the case, you might be able to save it.(re-flash the nand with the original files if you got em)
Also your downloaded content will have to be unlocked (Yaris Swap) to work on your jtag, as it is locked to the original 360 you downloaded to.
If you can't save your jtag (don't have original flash of the nand,...or e-fuses blown), you can remove the jtag mod, and then possibly buy a RGH mod-chip to run unsigned code again.
Also another option is that you could buy a device like the xk3y, or the wasabi 360 to play your iso's off of an external HDD.
Hope this helps