Can you post the make and model number please.
Are you getting the water light comming on?
Is the drum turning?
Has the dryer only just started to do this?
Is the water tray at the top empty?
Ok if you want to check yourself on this then you need to remove the top and the right hand side panel.
From there you will see the workings at the bottom. In there is the pump, Take this out of the chamber and give it a good clean out and then blow through the hoses.
Then retry the dryer and see how that goes.
Also check the filters aswell and make sure the condenser filter in the bottom left is clear aswell.
If this is all fine then it could be something to do on the heat pump side. As the dryers don't have a heater in there. They use a compressor like on a fridge and the condenser which generates the heater is blown into the dryer to dry the clothes. Which if this is the problem then this is a major repair thats if this is the fault.
So best thing to do is check the pump out and see what you find.
But these dryers are quite new so if its under 2 years old then it will still be under warranty so you will be best getting Siemens out to repair it if under 2 years.
If you need anymore info just get back to me.