What's the best icon to use, in order to indicate “Nothing Happened” or “No Data”?

To imply nothing happened / no data, I would use "nothing" - IE an empty circle:

warning sign and shoutdown logo

big red cross mark will be perfect, with subtitle of NO Data, or No File Found

use sheet or paper icon to point data and mix it with red x to point not available

you can use the empty word or you can use the zero sign with slash in the middle.

This is shown in a matrix of Scheduled Operations

the matrix has server names on the y-axis and dates on the x-axis

I need an icon that represents "No Schedule".

My first thought was,but I really don't feel like this is the right icon for "No schedule" or "nothing happened".

What would be the "right" pictogram for this in your opinion?

The data I'm showing is a matrix of clickable status indicators (the icons are the status indicators) - when you click an icon, you get details about the event.

My icons suite looks like this;

•  - Scheduled Operation Succeeded (Nothing went wrong)
•  - Scheduled Operation is In progress
•  - Operation missed its scheduled (basically as hard fail)
•  - Scheduled Operation Failed
•  - Scheduled Operation is waiting to start

you can use (!) or (?) in no data.

warning / notice will do the job 

Not applicable   N/a  also helps

better use a shape traingle puting (!) inside on it.

the best icon is the (X) icon