The answer is
In my case, it's related to the Toggle Vrapper Icon in the Eclipse.
If you are getting the bold black cursor, then the icon must be enabled. So, click on the Toggle Vrapper Icon to disable. It's located in the Eclipse's Toolbar. Please see the attached image for the clarity.
It sounds like you hit the "Insert" key .. in most applications this results in a fat (solid rectangle) cursor being displayed, as your screenshot suggests. This indicates that you are in overwrite mode rather than the default insert mode.
Just hit the "insert" key on your keyboard once more... it's usually near the 'delete' (not backspace), scroll lock and 'Print Screen' (often above the cursor keys in a full size keyboard.) This will switch back to insert mode and turn your cursor into a vertical line rather than a rectangle.
You might have pressed 0 (also used for insert, shortcut INS) key, which is on the right side of your right scroll button. To solve the problem, just press it again or double click on 'overwrite'.
This issue can happen not only in eclipse but also in any of the text-editor.
On windows systems, windows-10 in my case, this issue arose when the shift and insert key was pressed in tandem unintentionally which takes the user to the overwrite mode.
To get back to insert mode you need to press shift and insert in tandem again.
The problem is also identified in your status bar at the bottom:
You are in overwrite mode instead of insert mode.
The “Insert” key toggles between insert and overwrite modes.
This problem, in my case, wasn't related to the Insert key. It was related to Vrapper being enabled and editing like Vim, without my knowledge.
I just toggled the Vrapper Icon in Eclipse top bar of menus and then pressed the Insert Key and the problem was solved.
Hopefully this answer will help someone in the future.
Source: Stackoverflow.com