I have done something and now when I press the space bar to increase the space between words, it deletes the next letter

I have done something and now when I press the space bar to increase the space between words, It now deletes the next letter! could some one give me a clue how to stop this?

You probably accidentally hit the Insert key. That toggles between overwrite mode and insert mode. Press it again to cancel.

floris v It had corrected itself!! but I did try the Insert key and it did the same. At least I some idea if I do it again! Thank You