To put this in a simple way to start with, I would suspect that there are faulty components, or damaged soldering, or loose cables, or blown fuses (there are many fuses), inside the unit that need to be repaired.
Since you have not opened it up to check to see if there is any obvious damage, I kind of expect that you are not familiar with repairing damaged electronics. Perhaps you should tell us about your level of experience with electronics so we can see if we should try and help you troubleshoot, or just advise that you should get an electronics repair technician to fix it. It can be dangerous for beginners to mess with high voltages inside electronics if they do not know how to be safe.
Anyway, to start with something you can try without opening it up. Make sure you have set the input voltage correctly on the back of the unit to match the mains voltage in your area.
Found this service manual linked in another thread. May be of help to you or others here trying to help you:
http://www.eltest.hu/pdf/GPD3303SM.pdf