If the sideburn cutter has come off, it is because it is broken. You will need to replace it. You will find the item at shaveroutlet.com. See link below

http://shaveroutlet.com/shaver-replacement-parts/braun-activator-body-housing-with-trimmer-select-models-p-829.html

For instructional video on disassembling the shaver, also see shaveroutlet.com tutorial here:

http://shaveroutlet.com/video_tutorials