Hi. It means your bgl reading is higher than the display will show. Not sure what value is the upper limit on the SD but most meters give up at 33+ Don't want to worry you but I hope it was a faulty reading, have you repeated and got the same? I used to get this before I got my bgl under control.
You will need to test against a calibration solution from the Manufacturer to make sure the meter is reading properly.
Edit: i agree that if it persists then seek A&E asap. A chemist might be open tomorrow and some can do the test using their meter to compare,