this is just for people in future to find this, if they have the same problem which I had
I was using Aegisub to edit a subtitle file , and one day the cursor changed , so that I could not insert spaces or
text into the subtitle line of text.
Every time I hit the space bar , it would delete the characters in the text , instead of adding a space. Same thing
if I tried to type a new word into the middle of the text line.

This is a common problem with many programs. It has nothing to do with Aegisub, but with your keyboard
I went online for help and found the problem ., and the fix for it.
It was the keyboard mode. The keyboard can accidentally get set to 'OVERWRITE' mode , just from dropping the keyboard
or accidentally hitting a random combination of keys.
The solution is to make sure that your cursor is in the program you are using (if you do not have a flashing cursor active in the program affected, this fix
will not work).
With the cursor flashing active in the same problematic software, hit the 'Insert' key on your keyboard.

PRoblem solved.
Your space bar will now insert spaces properly, and you will be able to insert text.
thank you for your attention,
and I hope this helped you.