Fatal error: Maximum function nesting level of '100' reached, aborting!

Disable all plugins and custom themes and start troubleshooting to figure out what might be causing the issue. Start by working in a default theme (twentytwelve or one of the default themes that comes with WordPress). IF everything works, move on to testing your custom theme by enabling it. Does everything work? If the issue pops up on your custom theme, we know the issue likely stems from the theme and we can contact the theme developer for support. If your custom theme works, then we start enabling our plugins, one by one. After enabling a plugin test things. Does the issue appear? If it does, the last enabled plugin is the likely culprit. Contact that plugin developer and provide them with the steps you took to recreate the problem. In this way you can help figure out where the problem is coming from.