X3 Professional suddenly won't convert my project to a Wav file

It sounds like you aren't selecting anything to export.  Shift-A will select all.  Try that.

It's hard to start solving the mystery without knowing anything about your workflow.
Does this issue only occur in mix exports? What if you select a few bars of a single audio track for export?
How does a failed exports waveform look like if you import it back to SONAR (or in the wave player)?
Do you select every track for export or do you use the select nothing method (which equals to selecting all)? 
If the latter, do you remember to click in an empty space in the track field to remove any accidental selection you maybe have not noticed? 
Is everything routed through Master bus, and is Master selected as the export source?
Are these audio, MIDI or both? Do you have Fast Bounce enabled? Some VSTs don't like it. Is there some VST used in the failed exports that is not in the succesful ones?
Does the export include sample rate conversion?
Can you open an older project, already successfully exported, and export that?
I wouldn't mind seeing your export settings. Sometimes we miss an obvious flaw, even when we perfectly well know
how things are done. 
 

Hey! Thanks for the replies guys, I do appreciate it greatly. There is certainly much to consider here. Some of what you have said makes sense to me while some does not. I am no Master Masterer for sure, and have only been learning this in earnest for a few months. I go all the way back to Sonar 6 and had this failure to export thing happen once years ago with Sonar 6. Back then, like now, I did not discover what caused this. With Sonar 6 it was fairly easy-I just upgraded out of the problem and bought a more recent pc.
Everything was fine, like I said before, until about 1 week ago, when, suddenly, I couldn't export. After much deep painful thinking, I remembered that I gave Adobe PDF reader the right to update. I thought, " How could this hurt?" I guess I was wrong! I don't know what else could have occurred since I rarely put this pc online for more than a minute or so, and even then its only to Sonar no WWW.com stuff outside of a small handful of business related sites.
Anyway, I read through your suggestions, Kalle Rantaaho , and thought I would try the easiest one-using the Master option. It worked just like it should. I have only tried it on one song export so far, but it DID work. Happy me. However, not knowing from where it came or why, I wonder how long I am safe.
You are probably thinking, "What were you doing before?" Well, I have always followed Scott R. Garrigus and his books so that is where I get everything from. In his book, Sonar X3 Power! and all his books before this, I did what he said which was to Insert New Audio Track and then proceed to mute all tracks except the midi (Reason) tracks. Then, hit record. When finished, unmute muted tracks and proceed to export.
On exporting, here is what I did before which worked hundreds of times:
 
Export audio
Title song
Place in audio Library
Channel Format-Stereo
Leave sample rate at 44100
Change bit depth to 16
Triangular dither
Everything checked except Audible bounce and Live input
Then I check the 64 Bit Engine
 
After this I would go to Cakewalk content to find the song and play it. It was always there until now.
 
Update: I have just tried my old method again and it failed. I then tried the "Master" option, and it worked.
I don't understand why the old way suddenly won't work anymore. Fortunately, the new method works! But, like I said, for how long?
 
I hope this is useful info for you guys who are trying to help me. If not, then I will see what else I can add.
 
 
 
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Thanks for the reply Cactus Music I am drawing a blank here on "Select None". Where do I do this at? I mean where do I find this? If this happened to you before then I would like to try what you are doing if it gets me around this problem.
That's good advice which I have not tried yet on the opening up file in different location. I tried different players but not importing it back into Sonar.
 
Thanks guys for the speedy replies. Please let me know if I need to provide more info for you to help me solve this case.
 
bob