To repair parquet floor, identify all the loose planks, take them out, remove old glue from them, apply new adhesive, and put the planks back in their spaces. Fill in the gaps between the planks with floor filler, and sand the surface using several types of sandpaper. Apply stain and finish to the parquet.
After removing loose planks, number them on the back, and write the respective numbers in their spaces, so that you don't mix up the patterns when putting the planks back in.
It is a good idea to buy a rubber grouting float for filling in the gaps between the planks, as filling each gap by hand takes a lot of time. To fill in the gaps, pour some floor filler onto the floor, and use the rubber grouting float to spread it out. Latex floor filler is the easiest one to use.
Start sanding the floor with an orbital flooring sander with 20- or 36-grit sandpaper. Continue sanding with finer sandpaper. If the flooring sander can't reach some spots, use a palm sander instead. Apply stain to the floor using a rag or a brush, let it soak in, and wipe off the excess. Apply finish to the floor in several coats. Sand the surface with 120-grit sanding screen between applying coats.