I'm not suggesting you manually reconnect every missing track. The aim behind this bit:
Select a broken track in the library. Press CTRL+I to Get Info. Say no when prompted to locate the track. Look at the bottom of the summary tab for where iTunes thinks the file should be. Please post back the path as given. It will begin file://localhost/ and end with the filename of the track. Search the entire computer for a file with that same filename. If found where is it?
is to do some differential diagnosis. The information from one missing track would help me understand your problem better. Is the media really missing from your computer, or just not where iTunes is currently looking for it? If it is the latter I may have a fix that requires that you make a one character edit to a text file to fix everything. Or perhaps a drive letter has changed in which case I can explain how to fix that. Or I might want to suggest a couple of simple folder actions that need to take place and knowing the names of the folders involved means I can give you the correct advice. Or if the easy solutions won't work but the tracks are in sensible Artist & Album folders in some common location then my FindTracks script should be able to reconnect them to iTunes. FindTracks uses some fuzzy matching routines and searches for multiple potential locations. For more details see this thread.
With the number of tracks in your library it is obvious that you won't have a device that contains them all, but for those that do the user tip Recover your iTunes library from your iPod or iOS device (which is referenced within my first link) describes how to restore lost files from a device. This will normally require a third party tool to systematically extract all the media from the device.
If the files have truly been deleted from the hard drive, and you've no other backup, then recovering from the drive using a file recovery tool may be the last resort. In the past I've used FileRecovery from Seagate ($99) to recover some crucial video files from a friend's 500Gb hard drive that suddenly decided it was completely empty. From a fairly brief test it seems FreeRecover can do the same task, if not so elegantly, for the low low cost of free. Provided that the area of the drive that a file occupied hasn't yet been used then it is usually possible to recover a copy of the data. The scanning and recovery processes take time, but don't require constant manual input. What you are likely to need is a new drive to restore the data to or you may damage files that could have been recovered.
Songs that have been purchased from iTunes can be redownloaded from the store if they can't be restored in any other fashion. If the entry for the song still exists in iTunes then it will assume you have it, even if the link is broken. To recover these the entries need to be deleted first (without removing from iTunes in the Cloud). Then, after closing and reopening iTunes, it should be possible to redownload from iTunes Store > Quicklinks > Purchased > Music > Not on this computer > All songs > Download all.
Without understanding more clearly what has happened to your system it isn't yet clear what has gone wrong for you. The inital problem I addressed in this thread when the library file is corrupted following an upgrade is relatively common, though still rare in real numbers terms. Apple could redesign iTunes so that it was able to detect that something had gone wrong and automatically apply the fix I suggest (which is the same as one they suggest here) but they haven't yet chosen to do so. For more obscure issues where files might go missing I cannot be certain that the relevant team at Apple are aware of the problem or indeed how many different issues there might be. I'm vaguely aware of a potential problem when tracks are downsampled when put on a device but I'm basing this on anecdotal evidence in a few threads. It isn't something I've successfully reproduced. Without knowing more it isn't clear if iTunes is actually at fault in such cases or if some other issue could be the cause such as corrupted file operations which would be attributed to Windows or the drive itself rather than iTunes.
tt2
Message was edited by: turingtest2