It may be that you have the incorrect channel selected on the TV. Even if your box was not getting a signal from the aerial you would still get the welcome screen or menu screen visible on your TV.
Check that you have selected the correct source channel on the TV, usually called "AV" or "EXT". Check your TV's user manual if in doubt how to find the AV channel.
It is also possible that you have the wrong specification of Scart lead. All Scart leads used with digital boxes must be of the "fully featured" type. If the lead was not included with the box but bought separately this should be checked.
If your TV does not have a Scart socket then you can still connect the digital box to the TV with an aerial lead if it is has 'RF modulation'. Boxes with RF modulation can pass the digital signal to the TV via an aerial lead. However the RF aerial output from most boxes only passes the analogue signal through the box (known as RF loop through) allowing the TV to pick up the analogue signal.
Finally, check your box is not on standby or power saving mode.
If all this fails to help then you will need to contact your electrical retailer or the manufacturer of your equipment.