Renee
Welcome to sales and welcome to the forum.
Your question was about contacting previous customers. Before you begin realize your customer base has more than one component. First are the customers who buy from your firm on a regular basis and are happy with the level of service, your products, etc. I think you would simply need to contact them, using the method you feel most comfortable with, and introduce yourself and tell them they can expect the same great level of service they have come to expect. I would ask them what else you would be able to do in order to continue that great service.
There are also customers that for one reason or another have not purchased as much, as often or perhaps not at all. Before you contact them and get blind-sided I would make a list of those customers and ask your management team what the circumstance was that led to either underperformance or complete lack of business from that customer. Then I would devise a strategy, with the help of your management team, to overcome the circumstance.
Nothing works 100% of the time so do not be discouraged if you cannot turn all the customers in group 2 to your way of thinking.
Best of luck in your sales career.
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Hope it'll be good enough