Show the letter to your attorney. Hire one if you don't have one. Just like when you have a health problem and hire a doctor, now this legal problem means you need a lawyer.
Based on the limited information provided I believe it means the tech claimed he was injured while doing the job & they have paid and/or are still paying workers compensation benefits to the worker - and they want to recoup their payment from you (probably claiming some fault on your part caused the injury) If this happened at your residence then contact your homeowners insurance to investigate & handle this matter - if it was at your place of business contact your business insurance company. You can have a local attorney check it out for you. Good luck
This is not intended to be legal advise or as legal representation. I am a California personal injury attorney . Be aware that every state has its own statute of limitations; and statutes & case laws that govern the handling of these matters.
The technician must be claiming that he was injured at your home. The only way that the technician would have any sort of claim against you would be if you did something negligent to cause the injury.
You should send a copy of the letter to your homeowner's insurance company for them to investigate and respond.
A roundup of the best tips and legal advice.