The most common reason for returned broken GP32's is people using the wrong adaptor (like 9 volts) but its often very hard to accuse people of doing this as they deny it.
However, I have discovered one fried in this way will often still work from a correct adaptor only (batteries don't anymore).
So if you have a 3v correct adaptor and try it, turn the GP32 on and off a bit it might come back to life.
My collection of hacked up GP32s/overclocking experiment/etc. are all returned ones with this issue which i put to a decent use.
-Craig
www.gbax.com