I was using ios version 4 till yesterday, and I just upgraded the version. Some apps which I purchased before cannot be found in the app store or in the itunes purchases section. what is to be done??
If you had your iPad in sync with your computer, the app should still be in the "Apps" section of iTunes, available for you to synchronize with your restored iPad.
Alternatively, on your computer, select "iTunes Store", and from the "Quick Links" section on the upper-right hand side, select "Purchased", and then "Apps" (and then "iPad"). Some removed apps are still available for re-downloaded from that area.
Apps fall into four categories:
As written, it looks like the app you have in mind is in the last category. Your only hope there is to restore a backup copy from iTunes and sync that app over WiFi (if your iOS is new enough) or over USB cable.
As to the status of an app in my list above, the developer has total control over their desires to place, update or withdraw an app partially or totally. Of course Apple reviews these requests and can broker / take action above and beyond the developer.
If you fall into the last category where the app truly is not available, you have several options: