I derendered someone, how do i undo it?

[Kristina Niki]

I derendered someone how do I undo it?

[Echo Hermit]

Welcome to Second Life Answers, Kristina

If you derendered someone using the Second Life official viewer, then you can either teleport out of the area and back in again, or fly out of range, say 200m and back in again, or failing that, a relog should do it. 

If you derendered someone using the Phoenix/Firestorm viewer, you need to go into the menus at the top of the screen, look for Assets Blacklist  and remove them from it.