Start by noticing what is wrong. Empty boxes, unusual Latin characters and separated letter parts can have different causes.
Empty boxes or missing letters
Try a current browser and a font with Bengali support. If other Bengali pages look normal, the problem may belong to that particular file or website.
Latin symbols where Bengali should be
The document may use a legacy encoding, or its bytes may have been interpreted using the wrong character encoding. Keep the original file. Repeatedly saving damaged text can make recovery harder.
Letters that look disconnected
Try another current application and a suitable Bengali font. The text may also contain unexpected spacing or invisible characters. Remove characters only after checking their purpose.
A simple test
Copy this phrase into the application you are checking: আমি বাংলা লিখি। If it appears correctly, the application can display at least this Unicode Bengali example. It does not prove that an older document uses Unicode.
Understand fonts and encoding →