1. If you fully completed your online questionnaire & successfully submitted it, the status would have changed form "Awaiting Subject Actions" to "Awaiting Sponsor Actions" or "In progress". Very often candidates think they've completed it fully when they haven't & if they don't complete it within about 28 days from creating their account, it gets cancelled. If it didn't get as far as "in progress" then it's "timed out" and automatically cancelled.
2. If it got as far as "In progress" then changed to "Cancelled", the odds are your AFCO cancelled it - possibly because you failed a selection element, didn't fully complete the online form after creating the account, or because the AFCO have been unable to get hold of you by letter, email or telephone and have subsequently binned your application in the assumption you are no longer interested because they cannot get hold of you. (Happens a lot).
3. If you got as far as "In Progress" and your AFCO didn't cancel it, then possibly there is a significant issue such as financial problems or undeclared convictions or security issues relating to "subversive activities". (Oooer). If it was any of these issues, you'll already know about it as NSV will have been in touch with you, requesting further detail and effectively giving you the opportunity to declare all.
Some of the questions on the security application may seem to many rather ludicrous, such as: "Have you ever been involved in terrorist activities or plotted to overthrow the government? Yes/No" and I always imagine someone ticking "Yes" then thinking "No, wait....". Has anyone ever ticked Yes and meant it? Yup, more often than many may realise.