If you dream about an old place of work- it's usually your subconscious telling you to stop dwelling in the past.
Your old job is just that- your old job- it’s been and gone and rather than focusing on it- you might be better off channelling your energy into your current post.
On the other hand- if you dreamed about leaving your old job- perhaps your dreamscape is urging you to allow history to repeat itself and leave your current position for something better. You did it once- you can do it again.
If you have been in your current job for a while and you have had some uncertainty in that role- it could mean that you are pre-empting the change before it happens, such as a redundancy. If your gut is telling you to jump ship- it might be worth listening to it.
It seems that you might be holding on a little too tightly to the past. What was it about your old job that you remember so fondly? It’s possible you need to take the best bits and try and find another post that fulfils this criteria as it seems to mean a lot to you.
If you were offered your old job back in your dream, it could mean that you don't feel fulfilled or stretched in your new role and you need to look for something else that helps you to reach your full potential.
Another interpretation is that you don't feel there is any room for progression in your current position. Perhaps you don't like your job, feel that your managers are not invested in your future and you have been looking to move on for a while.
Dreaming about your old job might be down to a lesson you learned when in your previous role that you need to apply to the here and now.