iphone won't turn on - how to erase the data?

Does iTunes see it when you plug it in?

If you can't power the iPhone on by itself, and it doesn't power on when connected to the wall charger or when connected to your computer, the only way you can erase the data from the iPhone at this point is by opening the iPhone up and removing the flash storage.

No. The iPhone uses flash memory which is not strongly affected by magnets.

Try putting it in a bag of rice with the bag open and in a warm area for a few days (I used the top of a lamp shade for my old PDA - worked well). It might come back on after drying out.

You could also microwave it but that would reduce its Ebay value just slightly! 😉