I have several points and its attributes. I want to classify them into some class, but I want to classify it in line shape. When I convert it into line, the attributes turn out to be one (one row only) while there were so many attributes from the points. How do I convert those points without making the attributes into one attribute? Can anyone help me?
This is difficult to answer, based on the limited information in your question (eg, you haven't stated what process you used to convert points to line, or why you want to do this). However...
Assuming you want to create multiple lines and that you want each line to have its vertices based on a subset/group/category of points based on a common attribute value, you can do this using the QGIS plugin "PointsToPaths".
There's a really good post about the "PointsToPaths" plugin, including how to install it in QGIS, how to use it and some nice example screenshots of the results, at: QGIS Plugin of the Week: Points to Paths
However, if this is not what you are trying to achieve, and you really just want to keep all the points (every feature, and every attribute of every feature), and just visualise a line connecting each point feature, then you should convert the points to line (as you've already done), optionally delete all of the line's attributes that are not really relevant to it as a line (probably most of them), and then simply add it to your map, as well as the points (not instead of the points). Ie, view both points AND line on the map simultaneously.