What can I do about another company cashing my checks??

I have a small business and my customers payments have been cashed by another business that I was sharing an office with. The owner of this business told me that they had not received any checks belonging to my company. After speaking to a few customers I received copies from my customers of the cashed checks endorsed by this other company. They also have my computer with my business info and have admitted to viewing and printing my company emails. I have tried to contact the company again after I found out they cashed the checks and they still say they did not receive any checks. Now they do not respond to me at all.