I updated my jailbroken iPod to ios 6 and it didn't work. how can I fix it?

You can downgrade your ipod and jailbreak it again

to downgrade your ipod follow these instructions

-Download the "RecBoot" utility. Extract the "RecBoot" program files from the ZIP file to your computer.
-Open itunes
-Switch the iPod Touch to Device Firmware Update (DFU) mode. Connect the iPod Touch to your computer with the USB cable. Hold down the "Sleep/Lock" button to turn the power off.
-Press and hold the "Sleep/Lock" button and the "Home" button for at least ten seconds. Continue holding the home button, but release the "Sleep/Lock" button after ten seconds have passed. The iTunes software will display a dialog box alerting you that the iPod Touch is now in recovery mode.
-Click the iPod Touch in the "Device" menu in iTunes. Press and hold the "Shift" key on your keyboard and click the "Restore" button in iTunes. Select the iPod Touch firmware you downloaded earlier when prompted to locate the firmware file. Click "OK" to begin the firmware installation.
-Double-click the RecBoot.exe file you extracted earlier. Click "Exit Recovery Mode" in "RecBoot." The program will reboot your iPod Touch into the older version of the iOS operating system.