Say Earth was positively charged, and you converted negatively charged mass to energy, and back
Electric charge is conserved, so you can't convert only negatively charged matter to energy. You'd need some positively charged matter as well (ideally the corresponding antimatter).
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread is at almost 500 questions in about a month, so I think it's time for a new one.
Also, I have a new "stupid" question.