I’m using free version for couple months and now can’t use it since it shows it doesn’t have a valid license.
One reason could be that you have used the same activation key on another computer. The Free License is valid for one computer only, so you would have to register another Free License if you want to use it on more than one computer.
Another possibility is that eM Client cancelled your license because they suspect you are using the Free License for business use. In that case they would have sent you a message saying that they intend to cancel the license.