This indicates the probe was disconnected when the logger took a sample. The readings are paused until the next sample is taken and will show a temperature again when the probe is connected and the next sample is taken.
To start the VFC400: hold the start button down; the word "starting" will display. A second later the word ready disappears; when the word ready disappears you must remove your thumb/ finger within 1-2 seconds or it will go back to the "ready" state.
Simply hold the start/clear/stop button for 2 seconds and release. The temperature will disappear and the word "stopped" will display.
If the audible alarm is going off you've gone into an excursion and need to download the data onto the computer to see when/how it happened. Downloading and re-configuring the logger is the only way to stop the audible alarm.