for the original h2o, you can try the paperclip reset, as well as the manual factory reset that involves tapping both bottom screen corners while holding the power button down, you kind of have to grow a third arm for this move
( manual reset methods mentioned at the bottom of this page
https://wiki.mobileread.com/wiki/Kobo_Reset:_Aura_H2O )
as long as the power led still starts blinking there is some chance of success for these methods
if the power led just lights up w/o blink you may have to replace the internal sd card. opening the device is difficult as the front bezel has to be removed, but it's glued all around, and directly to the display itself, and the display is very fragile
you can also cut a hole in the back of the device for direct sd card access but if you cut to deep you damage the mainboard instead ;-)