If you hold back+ok for 30 secconds it should turn up as usb device in dfu mode and you should be able to reflash it from there, if it does not work after making sure you tried different cables and or pc just to make sure it is nothing related to local restrictions on a machine.
If it is in DFU mode, qflipper should give you the repair option. but it should turn up as a USB device if you hold the buttons long enough for it to go in panic mode reboot.
Also try a different old 512mb SD card or something like that to make sure it is not crippled partitioning or storage making it fail, but so far i have not managed to break it beyond repair so i hope there is a easy fix .