Mohit,
Its not very common but a few things could be wrong, including a cosmic ray nocking an important byte out of place. Try an empty.html and try and import from your wiki file and see if you can access the tiddlers that way. Don't import system tiddlers unless you created/edited them.
You could open the html in an editor and check to see if it follows the same patterns as in a standard empty file. You can at least confirm the content looks like a tiddlywiki file.
Of course if you can share the wiki it may be easier.
Regards
Tony