Is the tip of your soldering iron black, crusty, rough and ugly? Does it seem like it just does not want to heat up or melt solder any more? Clean the tip, and if that does not do the trick, then it is time to replace it.
To clean the tip, give it a brisk scrub with a small wire brush, or jab it into a blob of steel wool until it looks shiny again. Upon heating, get some solder onto it right away. Keeping the tip “tinned” is the best way to keep it clean. Wipe it on a wet sponge often during use, and keep melting a little onto the tip to keep it cleaned.
If repeated scrubbings have rendered the tip misshapen or broken, then just unscrew it and replace it with a new one.