Hi, There are a few issues which could cause this. Encase you dont have access to the manual a copy can be found at http://madamebutterface.com/assets/documents/Technics%Stylus-pressure%20adjusment%20knob-QD33%20Operating%20Instructions.pdf
Firstly is the deck level?
Does it make any difference if your adjust the Stylus-pressure adjustment knob at the rear of the arm?
There is a cueing button next to the left of the start/stop button, have you tried pressing this to see if the arm will lower?
Have you recently changed the cartridge or stylus? If the above still don't identify\cure the problem, its likely to be a slipped\sheared cog inside.
Unfortunately I cant find any engineer documents for this device, but if you remove the case I would expect to something out of place or damaged around the base of the arm. If you can provide any further details I will see if I can assist further.