have you tried it without the memory card in? If that doesn't work, can you remove the last (or last few) activity off it via the PC? It's usually a corrupted activity in my experience (not necessarily the latest one either)...
Tried the hard reset (power + lap buttons together) with and without memory card - no luck. Connecting the unit to computer with a cable brings it back to life for about 5 seconds.
have you tried it without the memory card in? If that doesn't work, can you remove the last (or last few) activity off it via the PC? It's usually a corrupted activity in my experience (not necessarily the latest one either)...