Is it happening with just one document, or with all documents?
If the former, it's almost certainly because the document has become corrupt.
Highlight its entire contents (Ctrl-A)
Copy it to the clipboard, (Ctrl-C)
Open a new document (Ctrl-N)
Paste the contents (Ctrl-V)
Close the corrupt document (Ctrl-F5, Ctrl-F4, No)
Save the new document over the corrupt one (Ctrl-S)
I suppose it's just possible that it might be a hardware fault (dodgy RAM, hard disk corruption etc) but it's far more likely to be related to software...
Thanks everyone but got someone coming out to sort my problems out.
Next question, Can i trust this person not to look at my personal details? Don't know much about the company just got a flyer through my letterbox the other month. I've been on their webite too.