I just had this happen. When you power cycle the amp back on holding A and B, you won't see any lights or anything on the amp. It will still look "dead". It is in bootload mode. Reopen Updater or Line 6 Central. It probably won't see the amp. It will give a message that says something about seeing a "generic USB device, do you want to identify it?". Select "identify". Select the product you need to update and just try the update again. Once the update shows as completed, power cycle at the end (no need to hold anything this time) and you should be good.
