Help! I Can't edit text on my website CMS

Hi! I have a website and use a Content Managment System to edit and publish content. For some reason I can no longer edit text? There is probally a very simple answer - I hope! So here's what happens: I can delete characters but then can't re-type. I can delete the whole of the text and then type but only for 5/6 lines before it freezes. I can delete the whole text and copy and paste but again, it will only insert the first 5/6 lines. I do not this issue when using a PC, so it got to be something I've done, or not done using my Mac. I'm sooooo frustrated, please help!!!!