Our graphic design company developed concepts for a company over a year ago. We showed all to the client and everyone was happy. Shortly thereafter, just as we were about to request sign off and send an invoice, the entire top management, including our contact, was suddenly tossed and replaced. We sent a few emails and waited patiently. And then Covid-19 hit, so nothing moved forward. No one from the client ever contacted us to complain or cancel the contract. We recently sent a reminder, requested payment and sent an invoice. The client now says "you provided no value", "there was no sign off by us so we owe you nothing", and "force majeure", even though our concepts were delivered before Covid 19 started. We have visual proof of the work and all we want is payment for our labor time. Thoughts or advice?