You probably lost the file-association to your Writer-documents.
Since we don't know which Operating system you are using, this is the Windows solution:
Rightclick on a random Writer-document.
Choos the option Open with... from the submenu.
Choose : Select program. A new submenu opens where several programs are listed.
OpenOffice should be amongst them, then select it, otherwise search for the program with the Browse button.
Don't forget the check the box "Always open this filetype with this program" (or something like that

)
below the listed programs, that ensures you that Writer docs always be opened within OOo