All you need is your broken/detached or loose makeup brushes and a hair dryer.
Put the brush back in place as tight to the wood as to the metal. Turn on hair dryer to hot. Blast at the metal part of the brush. By doing so, it will help heat and melt the glue which once held the 2 pieces together. Let sit to cool down. Once cool examine the brush to see if it's still loose or detaches again.. If so, retry process.. If this process is unsuccessful, detach the brush and blast the hair dryer to inner metal part of the brush to help the glue melt quicker. Use a tooth pick or a depressor stick to test how hot the glue is and move it to the sides of the inner metal to recoat it. Than place the wood end piece of the brush gently back into place and let sit to cool.
