The blade and tracks are controlled by levers connected to the control valve (spool valve) directly via rods and linkages.  The joysticks send electrical signals to the electronics which in turn drive the spool valves via electrically operated solenoid valves.  So if the joysticks aren't responding its probably an electrical/electronic problem and it can be difficult to track down the cause.  Here are some things to look for. 

Safety Arm.  If this switch is not working properly the joysticks will not respond to commands.  Check out the wiring as shown in the previous page and when the lever is operated 2 relays below the seat should click.  This is hard to hear when the engine is running so put your fingers on them and you should feel them click.  If this isn't the case check you wiring with a multimeter.  Best to check each wire for resistance rather than chasing voltage.  (wires that should be connected to earth often show voltage on your meter when they are not earthed)

Joystick Pattern Switch.  There is a multi position switch below the seat that allows you to select the joystick pattern.  This switch can become faulty and stop the joysticks working.  Below is a YouTube video from KR3W Jones for a Mitsubishi MX55 which had this very problem and details how it was fixed/bypassed.   

https://www.youtube.com/watch?v=p7QrDQri01g

Error Codes.  The electrical manual for the MM50SR gives details of the error codes and how to investigate them.   There has been some good information on the Heavy Equipment Forums from T-in-T about this and how to test voltages from the joysticks using the test points.  I've attached a link to these discussions which is well worth a read before you do anything else. 

https://www.heavyequipmentforums.com/threads/mitsubishi-mm30sr.7464/page-11

If all else fails.  Some people have given up with the electronics and converted the hydraulics to all manual controls.  "Life at the end of the road" website gives a good insight into how to do this using cables links to new joysticks but keeping the hydraulic spool valve block. 

https://lifeattheendoftheroad.wordpress.com/tag/mitsubishi-mm30sr/

Also, Jim Hardy on the Heavy Equipment Forums converted his machine to cable controls.  Below is the link to this. 

www.heavyequipmentforums.com/threads/cat-mm55sr-excavator.14252/page-3#post-1194068 

New Computer.   There is a recent post on the Heavy Equipment Forums from Oldironrevived about replacing the computer on his MM40SR.  It was supplied by I M Dynamics using their MM-R16-3 Universal 8 axis excavator controller.  See the links below:

https://www.heavyequipmentforums.com/threads/mitsubishi-mm40sr.68292/page-2

https://www.imdtech.com.au/products/