While sketching, I rotated the viewpoint accidentally which now stands no longer normal to the sketch plane. How could i return it as original?

Select "Normal View" on the Veiw toolbar  so that the viewpoint will be back to original. If you select it once more, the viewpoint will be opposite.

. How to work with mouse in CATIA V5 ?

1. Panning - to move the model on a plane parallel to the screen.

Click and hold the middle mouse button, then drag the mouse.

2. Rotating - to rotate the model around a point.

Click and hold the middle mouse button and the right button, then drag the mouse.

3. Zooming in/out - to increase or decrease the size of the model.

Click and hold the middle button, then click ONCE and release the right button, then drag the mouse up or down.

In CATIA V5 the tree disappear while working, I cannot reset it back even after pressing F3. Why?

   The tree may be too small to see, like a dot on the screen. This will happen when you activate the tree and zoom it out too much.

First, activate the tree by clicking the white coordinate system at the lower right-hand corner.

 Then click "Fit all in” icon present in the view toolbar.

How to reset toolbar when disappear from Catia V5 work bench. Why it happens?

Every toolbar has a "X" button on its upper right corner. If you select it, the toolbar will be closed.

If you  Select “View > Toolbars”(your selection)

The list of current toolbars will be displayed. Currently visible toolbars are indicated by a tick symbol to the left of the toolbar name.

From  the list, click the toolbar you want to view or hide.

You can detach toolbars from the application window border by dragging the double line to the left of the toolbar. You can drag the toolbar anywhere around the screen, then dock the toolbar in the same or in another location by dragging it onto the application window border.

To restore the original positions of the toolbar on the current workbench, 

Select “View>Customize>Toolbars>Restore position”.

Sometimes the toolbar may be hidden at the lower right corner of the window. If you see a double-arrow icon at the corner, it means more toolbar are available. Drag out the other toolbar so that more can be seen.