Hello Julian:
This should be easy to troubleshoot. Feed a signal into two Channels of your O'Scope (a working channel, and the non-working channel).
Configure the working channel for a proper display of the signal. Then configure the non-working channel settings the same as the working channel. If you don't have a convenient signal source to use, them simply connect both channels via their probes (using their clip-on tips) to the Calibration signal Test Point on the front of the Scope (provided for 'probe compensation adjustments'). Now using a third working channel trace the signals through the two channels (the working, and the non-working). Trace through both to see where you loose the signal in the non-working channel.
The video that xrunner generously provided should help you understand the oscilloscope's Front-End circuitry, so that you know where you are, and what is that you are probing, etc.
Hopefully you should be able to determine the problem and apply appropriate corrective action to get it working. If you have any questions I will try to assist you.
Good luck, Ted