Today, an advertisement has ran for my web, and I found that visitors were more than 50-200 people at the same time. I dont think it was overwhelming traffic though, After then, whenever click the pages and admin pages as well,[color=#ff0000] Error occurred [/color]popped up.
Our customers had difficulty with proceeding check out as well today.
For reference, we use ' bluehost ' for web hosting.
Anybody knows why? and could help me out??
Best is to ask your host first and look in your logs. Probably your database couldn't handle the load or something like that.
I will agree on that. Most certain is that if your store is more than year online, you have a huge database because of the stats tables. I personally clear those tables every 3 months.
Check this out. I had a store with 356 MB database and after I cleared those tables it went down to 65MB! Huge difference .
In this case, size doesn’t really matter… 
If you’re having 50-2000 simultaneous users then you’ve probably completely swamped your underlying server. If the server (or one of the key services like httpd or mysqld) died or needed to be restarted then you probably have a corrupted database and will need to review it and repair/optimize tables as needed. That your host kept you on line in this case is something to be thankful for.
Most dedicated servers could not handle that many simultaneous users on any single system. But I’m going to guess that you were looking at the “Online users” count which is missleading and represents total number of new sessions over a period of time.