I got hacked last night and spam emails were sent from my account. I can no longer send emails and by business has ground to a halt

Came in this morning and my inbox had so many emails of return to sender from porn sites etc. 
someone had used my domain name to send I don’t know how many emails. I cant use my email as have exceeded my daily quota and cannot communicate with my clients

Best option is to immediately login to your email account using the web interface for your provider and change your account password. Then discuss with your email provider what can be done about this as it has nothing to do with whatever email software you are using on your computer. You could have been using Thunderbird or Outlook or whatever.

If you feel that eM Client is responsible, as a Pro Licence user you can make use of your VIP support option and open a support ticket directly with eM Client.