We took off toilet from carpeted floor and the flange did not seem to be attached to the concrete subfloor at all. All the parts came up with the toilet. We have installed hard wood flooring to the concrete and when we place the flange over the drain, there is nothing but space where the holes are. How do we correctly install the toilet?

So you have a solid concrete floor in this room, right? Is the closet bend set into the concrete or is it set in gravel under the slab?

You can buy a replacement flange that sets down into the old pipe. Before you set the flange, if you can, hammer 20d galvanized nails down on an angle to give you a hold point. Then, get some anchoring cement at HD or Lowe's, Put some long screws or lag bolts through the flange screw holes, set the flange and fill the void with the mixed cement-it sets up fast! When it is set, you should have a pretty solid flange. Use the toilet bolts as usual.

The toilet flangeMUST be connected to the closet bend. Either with a inside closet flange or a glued or caulked joint. If the flange sets under the wood floor it can be brought to grade by flange extenders, (see image). Good luck, Tom

Yes, the floor is concrete slab. I'm going to assume the closet bend is set in gravel under the slab because the shower drain was.