Disappearing Waves

EDITED
A major issue just happened again: waves are missing from my project.
 
• Two tracks of soft synth guitar (Fab Four) had been bounced down.  The waves are gone.  Thank God I still have the MIDI, although rework is required since I used Melodyne to stretch the last notes.
• The Waveforms for the recorded vocals are gone.  What's in Melodyne plays, but the original waves are gone.
• An external organ I recorded is gone.
 
I previously said all the corrupted tracks had effects.  That was incorrect.  I now see ALL AUDIO has disappeared.  In all cases, I just see a straight line where the wave used to be.  This tells me that I did not somehow accidentally erase these, in which case there would be no clip at all.  In all cases, these were fine for months and then suddenly disappeared.
 
This has happened to vocal takes before.
 
Note: In this same project, an audio track that receives audio input from an external synth is silent and not metering.  I can see the synth metering on the interface.  The receving audio track has the correct inputs and outputs--but nothing.  How the hell does this happen?
 
People used to tell me to bounce tracks for safety in case the synth was not available.  Ha.

It's unclear to me if you're missing only the waveform or the audio. You mention "all audio" and then refer to the flat line. But as the visual appearance is not in connection to having/not having audio, I'd like you to confirm that you are actually not hearing any audio on playback. How about the audio folder? Are they there?

Hey, guys.  Sorry I was not clear.  Here is the deal:
 
Tracks that were actually recorded or frozen (e.g., recordings of vocals or external synths) make no sound and show no waveforms.
Tracks using soft synths (e.g., RealGuitar, EZ Drummer) that are not frozen are fine.  The ones that were frozen have to be unfrozen.
Vocals that have Melodyne can only be heard in Melodyne.  The original vocal underneath Melodyne is gone (that is, flat line wave and no sound).
 
Basically, if there was a waveform in the track, the waveform is flat-lined and there is no sound.
 
This is not the first time this has happened, but it is the most egregious.   That means "the worst."

I assume that at one point the audio for all these clips was both visible and audible. If so you could have disk or file system corruption or even a virus could cause strange issues this this.
 
Before you do anything else I would suggest that you really check your settings for these clips.
Find out the name of the file associated with the clip by right clicking on the clip and choose associated audio files. from the drop down. Then try to import that file into an empty project to see if has audio and a wave form. This will tell you if the file is ok. If so then the project is corrupt, if not then we keep looking.
 
If your computer connects to the internet at all I would suggest that you scan for malware using something like combofix from bleeping computer. just google it and run it.
 
Also run a chkdisk on you audio drive by opening a cmd prompt and typing "chkdsk [drive letter]: /f /r" it may ask if you want to run it on the next reboot if the drive can't be locked. Be advised this can take a long time so you may want to run it overnight or anytime when you can leave the system be till its done.
 
assuming you do not find any malware or disk corruption you may want to try a different keyboard. I have had really strange things happen when there was a key stuck on a keyboard.
 
as others have suggested it is possible that the memory could be having problems.
 
Good luck