If your cello bridge is not set up, or has fallen down, don't panic. This article and video will walk you through setting it back up without any problem.
The first thing you need to do is look inside the A string side F-hole and see if the soundpost is still standing and in place. That F-hole is the one on your right if you stand the cello up and face it. (on your left when playing it)
If you find it has fallen, you will need to take your cello to a local luthier to get it set back in place. They have the proper tools and know-how for proper location.
The bridge should be placed with the feet centered between the f-holes and in line with the notches in the F-holes.
Once you establish your soundpost is standing and positioned ok you will want to get the bridge set up. Most of the time the tailpiece is on, but wrapped in packing and the strings are wound tight to keep things under tension. When you unwrap the tailpiece, make sure you keep some soft material under the tailpiece to prevent you cello top from getting scratched. Loosen the strings enough to allow the bridge to be put underneath. It is a good idea to put some pencil on each string groove of the bridge and the nut before placing the strings on them. This helps the strings slide across the top while tuning minimizing the pulling of the bridge top.
The lower side of the bridge is the A string side. Make sure the back of the bridge (the side facing tailpiece) is square with the top of cello and the feet are flat against the top.
Make sure before you get tension on the strings that the tailpiece adjuster is in the correct place on the endpin fitting and centered on the saddle.
While you tighten the stings start with the A and C (the 2 outside strings) then follow with the D and G (the 2 inside strings).
When you tighten the using the pegs you have to apply steady pressure, pushing the peg in while turning. They will slip if you don't.
Don't go crazy on the A and D strings they break easily! you need to keep checking the bridge feet while you tighten the pegs and make sure they are flat against the top. If you find the bridge top is pulling and the feet are lifting then straighten it out by placeing your thumbs at the top and your other fingers firmly supporting from the back, push slowly and firmly, but carefully until the feet are sitting flat again.
It is best to have an electric tuner so that you don't over tighten the A string. When you get to G# , stop and use the fine tuner. If you go past A# to B you will break your A. If you didn't, you got lucky! It doesn't take much to over tune especially using the pegs.