Sharepoint Application Offline with no clues, App Pool Restarted to make it online again

Hi,

1. Our Sharepoint 2010 application went offline all of a sudden.After restarting application pool, application was working fine.We tried to find the root case from log files(Event and ULS) but no clue. Any thoughts as in what has caused this issue.
2. Also can you provide your thoughts\references to answer below questions:

• Since there is no log in this case for such issues how can we perform Root Cause Aanalys?
• What can be done to prevent this situation happening in future?
• What Preventive maintenance we should do to prevent this happening?
• What alerts can we set in order for us to get early warning?

Thanks, Rakesh
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