mail app shows when I have new mail but has blank page when opened
Mail app does not work - shows new mail on desktop icon but blank page when opened - deleting and reinstalling hotmail account details does not work
iPad 2 Wi-Fi, iOS 8.4.1
Try Settings>General>Reset>Reset all settings. The device will reboot on its own. You will have to enter all device settings again including network passwords. No data is lost when you do this.
IF that doesn't work, restore your device to factory settings using iTunes. Follow the instructions carefully. Backup first.
Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support