A quick search for that Registrar and domain name returns results for their WHOIS contact info complete with telephone number and email address, plus their control panel login page, so those are good places to start.
Before contacting them and pursuing this further, it would be smart to do a quick WHOIS lookup on 'your' domain to confirm you have the details correct. If the former employee bought the domain then it is possible they own it and not yourselves, though you would have a strong case if you planned to file a dispute with the Registrar.
Worst-case scenario is to file a dispute with ICANN but these are difficult and not really geared to resolve customer complaints. You would need to document your attempts to contact the Registrar and provide evidence that you represent the business, and that the Registrar is not merely slow to respond but is failing to take appropriate action to resolve the issue. This is complicated and since it's your business it probably means hiring a lawyer.
Make an extra effort to contact that former employee because that's the easiest way forward. After that send the Registrar a proper business letter (and obtain proof of delivery e.g. use FedEx or DHL or UPS, etc...) which conforms to their dispute resolution policy. Failing that, it's 'hire a lawyer to sort this out via ICANN'. Good luck.