Please help me!!!!!!!! This is so annoying?

ok, i'm not sure if i am going to word this right but, i have the open office word processor, and when i typed a sentence, and wanted to add some thing else in the middle of the sentence, i placed the curser in the appropriate spot, and when i started to type i noticed that the letter after the one i just typed was deleted, and obviously this continued to happen causing me to have to retype the entire sentence. i know that originally it did not do this so it was something that i did, which changed it, and i dont know how to change it back. so obviously my question is how do i change it back? any help would be greatly appreciated, thank you.