You may have discovered the hard way that leaving your broom standing against a wall can curl the bristles and make the broom difficult, if not impossible, to use. Outdoor workers often discover this after leaving a push broom standing against a deck railing in the rain. It usually isn't difficult to straighten the bristles, but it depends on the types of bristles. Some natural fibers, such as broomcorn, can actually break if you try to straighten them. This seldom happens to all the bristles, though, so you can usually still restore the broom to top-notch condition.
Run a wire brush lightly through the bristles to search for broken ones. Snip these off with a pair of scissors. The wire brush also cleans the bristles in preparation for steaming.
Bring a large pot of water to a boil on your stove. Hold the broom over the boiling water until it gets saturated with steam, which may take from 3 to 5 minutes. Move the broom back and forth to make sure all the bristles are equally exposed to the steam.
Remove the broom, let it cool for a minute or 2; then run the wire brush through the bristles. They should be flexible enough to straighten when you do this. If not, treat the bristles with more steam.
Hang the broom from a hook to let the bristles dry after you've straightened them.