Well, to recover it from where it is, you need to perform a hard config reset.
To do that, there should be a small hole right next to the serial port that you can stick a pin in. Press that for a few seconds when it's powered and it should reset the config.
If that doesn't work, try using the
recovery tool (click to go to the post and download the software and needed files)
Once the process completes, the switch should be operational again.
Finally, when upgrading going from 1.5.0 to 1.5.6. So many changes have occurred that the config file is likely no longer compatible.
This is why we typically recommend you upgrade and then backup the config. Or, keep up to date on each software version so changes between config restores are minimal.
That being said, what might work is this:
once the switch is restored,
you can downgrade it back to the original version of firmware,
then apply the config,
upgrade to the latest.
If that doesn't work, then you will need to restore the switch again and upgrade it back to the latest and simply manually re-enter the configurations.
However, by far the the safest way would be to recover the switch with one method above, make sure you're on the latest firmware, and then simply manually enter configurations.
Hope that helps.