This might be happening because you might have exceeded your 30/60 day trial if it is Microsoft 2007.
If it asks you for a 25 letter code when you open Microsoft, you will probably find the code in the booklet or in another booklet that came with the laptop/computer.
Try the code, this should work.
Hope this helps!
Okay specifically with Microsoft word 07 right?
There could be one of two things wrong with it.
1.) You have reached the end of the Microsoft world Trial. Which means you need to buy Microsoft word to continue using it. You can buy the actual version on here. http://office.microsoft.com/en-us/default.aspx
2.) Your probably trying to edit a protected document. If you want to edit it you need either a password for the document or get the original file for the document. If none of these apply then you can try and contact Microsoft about this. I hope this helps