When I go to BanglaWriter, it shows an error message: Not Implemented.
This condition too has been removed or resolved. You will notn hopefully experience this problem ever again. However, this is what you need to do should it ever raises its ugly head.
When you first enter into any page in BengalOnline please allow sufficient time for the browser to load everything completely (i.e., until the Done message is displayed in the browser status window) before you select the BanglaWriter.
Time is required to allow all program codes to download and embed. If you select BanglaWriter before then, you get the 'Not Implemented' error. What happens is that the codes are not downloaded and embedded fully and you are trying to use that.
Click the reload or refresh on your brwoser menu after the page has fully loaded.