Hi Santhosh,
1. Since "available.log" doesnt have more info, you need to check in the file std_server0.out of /work directory for downtime info. Also see dev_server0 file for related error.
2. You can restart your JAVA Instance alone from "SAP Management Console" (SAP Management Console can be accessed as http://<servername>:5<instance number>13
Regards, Praveen