Accidentally pushed the drum-tray back in without the drum? The green light won't show, and the tray won't budge - all you need to do is to carefully push a tiny lever below the tray, as pictured. 

good luck

What to do if you have drum stuck in your RP 3700 that is not in the right position.

Original article: http://www.drucksalon.ch/828/how-to-remove-a-stuck-drum-from-a-riso-rp-3...

Procedure for converting the color of a Z-type drum. Steps 1 - 9 also work when machine does not recogonize a correctly installed ink tube.

There are several Risograph Z-type drums, most of which are not universally compatible.
Use the following charts to determine compatibility with your machine:

North American Models

RZ 590 & 390 Compatible

MZ 790 & RZ/MZ 990 Compatible but Drum Code Entry may differ
         MZ 790 Drum Code Entry (Test mode #1210): 69
790 can be converted to MZ 1090 with special kit