If you're looking for a new job and want to see if you can return to a previous job, it might be the time to reach out to an old boss. If you are no longer in contact, you'll have to do some digging. A word to the wise, however: It's always a good idea to maintain good relations and contact with employers with whom you had a good relationship, as they could be your best allies when you're looking for any new job.
Poll Your Acquaintances
You might not be in touch with your old boss any longer, but you may still have co-workers who are, or even ones who still work for the boss. This can help you in two ways; first, you'll be able to get your boss' number or email and address from those former co-workers. Second, the co-workers who still work for your boss might also be able to put in a good word for you.
Use Social Media
Thanks to the popularity of social media, it's often easier to find long-lost people than you might think. LinkedIn might be a beneficial resource -- and if your boss is interested in making connections with other people in the industry, chances are she has a profile there. Also try searching on sites such as Facebook, Twitter, Instagram or others. If your boss has a common name, narrow down the results based on the city, state, or industry.
Call the Office
If your boss is still working for the same company, it might not be totally out of the question to simply call his office and ask how to get a hold of him. The company website can be another helpful resource, providing you with his work email address, at the very least.
What to Say
However you manage to contact your old boss, don't assume that she's going to remember the details of your employment. Call her, send her an email or a hand-written note, but however you do it, add in some detail to jog her memory. Remind her of your past position, how long you worked for her and mention a few of your biggest accomplishments. Then let her know directly that you're hoping for a new position within the company, and ask for her support.
If things go well, ask your boss out for coffee. This gives you a chance to take part in an informal interview before the formal process takes place -- meaning you'll need to be prepared to talk about why you're best for the new job. Another thing to keep in mind: whether you'll have to resolve any past issues that precipitated your departure the first time. If your boss cited you for specific issues in the past, be prepared to talk about how you've solved those issues or how you'll deal with them moving forward.