Yesterday I created a video in Sony Vegas.  I could click on the play button and watch it in my preview window.  Today, I open the same file.  I click the play button and it will NOT play!!!  It just sits there and does nothing.  I can open a different file and it plays just fine.  Then I come back to the one I really want to work on and it will NOT play!!!  
What is WRONG???   This is nuts!!!
P.S.  I cannot believe all the junk I just had to go through to register to post this question.  Good grief!!!!