Suddenly my outgoing emails are not appearing in my sent box, I've checked all other boxes, they are not there.

Try Tools -> Settings -> Mail -> Send

Check ON "Save copy of sent messages in “Sent” folder

Hi Sandy,
please do try what Steve recommends above, and if it does not solve your issue, please share more information about your account set up and which version of eM Client are you running.

Regards,
Olivia