Have you lost the signal bars with the cover closed? If so, I think, from what I have read, that it means that your phone has no service (has lost signal), when the cover is closed.
The way to prove this is to try and call the phone, (with the cover closed and you see the symbol) and see if it rings. If it doesn't ring, that proves that it has no signal.
If that is the case, the cover must be masking the aerial somehow so that it loses signal. There is no metal in the cover acting as a shield is there?
Here is what I think it means (2nd time around- hopefully got it right this time)
This symbol (a circle with a horizontal line in the middle - I think that yours is with n/a - it varies depending on phone maker) means that you have enabled the interruption mode and set it to "none." This means that you will not receive any notifications such as calls, SMS messages, alarm tones etc. on your smartphone. To disable this mode on Android lollipop, you simply have to do the following on your smartphone: Pull down the status bar with two fingers and tap on the button with "None" or the circle symbol with the line (or n/a) in the middle. When you have touched the button, then the mode is switched from "None" to "All". The symbol disappears in the status bar and you get back all notifications on your Android lollipop smartphone.