1. All you need to do is to reset the handset.

2. Simply unplug the phone (by pulling out the power cable on the phone and leaving for 20 seconds). 

3. Then plug the power cable back into the phone and it should restart the phone again.

4. Please check how you have connected the phone up to your network?  If using a Gigaset Deskphone please check that the ethernet cable is connected into the back of the phone into the 2nd socket as shown below: