How could I be banned if I haven't asked any question for a long period of time?
The amount of time is irrelevant. The ban is based on an undocumented formula that weighs your contributions to the site and if it drops below a certain threshold, your account is banned, and as soon as it rises above that threshold it will be unbanned. One of the few tidbits that we know about the actual formula is that time is not factored into the equation.
But it doesn't describe my problem, I've asked 15 questions where only 4 where down voted.
Few of your questions have upvotes (or at least a positive score), so that is likely factoring into the equation, as are your 4 downvoted questions.
I also have 2 deleted questions.
This is where your problems probably start. I see you have the Peer Pressure badge, so it could mean you deleted at least 1 heavily downvoted question. If you did delete a question to earn that badge, those downvotes are still factored into the aforementioned formula.
Assuming that the 2 deleted questions you mentioned are self deletes that you know of, do you know if you have any moderator deleted questions as they will also be a factor.
On the surface, your profile does not seem like a typical question banned user, but the hidden info in your profile that we can't see
- How heavily downvoted your deleted questions are
- How many deleted questions you actually have (including moderator deleted questions or questions that you deleted a long time ago and have forgotten)
are going to tell the whole story.
But the best advice anyone can give you is to improve your questions - not just your downvoted questions, but any or all of your questions. If you start getting a few more upvotes, you'll probably tip the scales back in your favor and will get unbanned.
As you have indicated you had the capability to ask a question yesterday (which you found out by pressing the "Ask Question" button and did not get the ban message), but discovered today that you were now banned (by repeating the same process and getting the banned message). Assuming that this is the case (which I can't confirm, but I have no reason to doubt this is what happened), the only way to explain it is some sort of change to the status of your account.
The ban is calculated on the fly when you try to ask a question, so if you pressed the button yesterday and it worked, but when you tried it today and it didn't work, it means something changed between the time between your question attempts:
- Additional posts of yours have been deleted. These could be moderator, community, or self deleted questions. Your recent deleted questions only shows questions asked recently, not deleted recently, so if a mod (or the Community user), or 10K users deleted as old post of yours, it won't show up in that tab.
- Your have new downvotes on your account. Doesn't look to be the case, but wanted to mention it.
- Posts that you have answered and were deleted. the delete wouldn't be the trigger here, but the loss of rep might be to blame.