Try this. I am not sure if the 990 has the same bootloader as a 890, or the same process, but its worth trying. This is the process on the 890 so hopefully the same:

To enter Bootloader mode, remove battery. With battery removed, press and hold Power On and Mute (keep holding both down). Reinsert battery and depress buttons. You will get a message that the files are not found and to connect to a PC and download. Before doing this, remove battery and reinsert. It should find your files.