First, try contacting her "indirectly"--that is, speak with someone else who knows her or who works with her. It's not impossible she is having a personal or health crises and that's why she's unresponsive. If that's not the case, you can bring a legal action in probate court to ask the judge to remove her as co-executor because she is not discharging her responsibilities. Because this is an expense of the estate, the estate should be able to pay for the legal action.