If i try to run sales report my website and backend not working, every time i am going to reboot my server. Are any possible methods exits to avoid this issue?
You are likely exceeding the maximum available server resources needed to handle the task. It is not that things are not working, but rather the server has begun queuing incoming requests and/or queries while processing what it can. There is a good chance that you can reduce the amount of required resources by ensuring that log tables do not contain more than a few days of data. In the backend, find the setting
System > Configuration > Advanced > System > Log Cleaning
It is likely set to 90 or 180 -- which is kind of disturbing to me due to the significant downfall of performance. If you set it to 3 days and it contained tons of records, you'll have to wait forever while the cleansing is done (meaning to run it at a very quiet time, so not to affect customers experience).
If the cleansing never finishes, there is a slight bug in causing it to choke on one of the SQL queries.
See this article for resolution: http://blog.kirkmadera.com/magento-customer-and-visitor-logging/