We need a little more detail to look into this. Drafts are normally removed once a reply is posted, so we need to know how this reply was made to find what kept it.
Did you post the reply from the quick reply box at the bottom of the thread, or from the full reply page (More options)?
Was the same thread open in another browser tab or on another device, with the reply text still in its editor, when you posted?
Did you post soon after you stopped typing, or on a slow mobile connection, and did the post seem to take a while to submit?
Has this happened more than once, and if so, does it happen every time you post from this device?
This is an automated assessment. A member of staff may review and update it.