Has your city's overall education level increased recently? Low level industrial buildings want uneducated workers, and workers will prefer to take jobs that are of their education level. Your workforce may be over-educated now and avoiding industrial jobs.
If this is the case, the only way I know to deal with this short of shuttering all of your schools is to keep a close equilibrium between workers and jobs. Educated workers will take lower education jobs if there's nothing else available, so having a shortage of jobs at their level will force them in.
Once you've done that, start working on raising the level of the factories themselves. Higher level industry takes higher level workers and will be much happier in your city with its educated work force. You can do this by adding services to the industrial area like fire departments and transportation (particularly freight trains).