Rollover in the stock market refers to carrying forward an existing position in the futures contract from the current month to the next or far month contract, just before expiry.

Yes, margin is blocked as per your position. SL reduces your risk, but initial margin is still needed.

For SL-Limit: The limit price was not reached. For SL-Market: Market closed or volatility jumped price beyond trigger. You may also not have sufficient margin or holdings.