How can I get my NXT to turn on - all it does is softly click?
If you are using NXT-G, the software that comes with the NXT set, then here is what you do: 


Get into NXT-G, start a new program.
Select the tools option, and from the drop-down menu, select "Update NXT Firmware..."
Plug your USB cable (one comes with the set) into your NXT. Don't try using bluetooth, as it won't work.
Click the download button on the interface which popped up at the end of step 2.
Don't unplug your NXT, wait until it reboots with its firmware loaded.


Congratulations! You have just brought your NXT out of SAMBA mode!