After ticket re-opened by an analyst, SLA should start automatically.
Please help me to resolve this.
Unfortunately it does not restart automatically on its own. When a ticket is closed, automatically it will close all events. So if you change the status from close to open (or any other "open" type status), the system will not regenerate or start the timer from last event recorded. If the Priority is changed, it will most likley kick it off again depending on how you have it set up. If the ticket is changed from closed to open with no action (meaning no changes such as "priority") then the SLA won't start automatically. This is by design.
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