Did you attempt to update your firmware, but it failed and now your Applesauce is unresponsive? If so, this document will help you to unbrick your Applesauce.
When your Applesauce is first plugged into USB, it will have a bit of a light show on the front panel as it initializes itself. If you don’t see this light show, then the firmware isn’t running. As a secondary test to rule out it being an issue with the USB on your computer, you can also try using a phone charger block or similar that plugs into an A/C outlet and has a USB plug that provides +5V. Applesauce will perform the light show when plugged into one of these. If you see the lights, then it isn’t likely that your Applesauce is bricked.
A firmware update typically fails due to not enough power being provided by the USB port that the Applesauce is plugged into. The failure will typically have a message like “Failed while uploading block 7!” Applesauce requires a bit more power than it typically consumes when it is reflashing the firmware, if it can’t get that power then things fall apart.
The ability to unbrick is part of the design, and I have never had an Applesauce that couldn’t be recovered after a firmware update failure.
First off, it is very likely that your USB plug isn’t providing enough power. This can often happen with older computers. It would likely be a good idea if you kept your Applesauce plugged into a powered USB hub. They are widely available and pretty inexpensive. It is also possible that a different USB port on your computer provides a bit more power, or some monitors have built-in USB ports that can be powered.
You will likely also need a Philips screwdriver (#2 size) to open up your case.
Here are the steps to take in order to get things working again. Be sure to follow them in order as it matters.