I'm always surprised when clients don't revisit their old clients or customers as a way of generating business. These people already know and trust you, so why wouldn't you? Often you just go off their radar.
Call or send a reactivation letter, email or text reminding them of who you are and what you can offer them. (Assuming you have your GDPR sorted of course!)
Ask how you can get their business back. Offer them a great incentive to buy from you or use your services again. Quite often a simple phone call or text will bring in new business.