First: this has nothing to do with InDesign, it has to do with your OS, so second: which OS? Windows or Mac because to solve this we need to know which one. It is your OS that has lost the association with your document files and program.
Thanks for getting back to me.
its OS X Yosemite 10.10.1
i am on a MacBook Pro 15 inch
IVe been using Macs for over 20 years - never had anything else.
select a InDesign Document in the Finder;
type Command I. (To get the info panel)
Select Open With
Choose the correct InDesign App
In the Info panel set Open all documents with this app.
it was already set to open with InDesign and although I selected it again it hasn't made any difference.
THanks for trying.