IPod Touch 4g upgraded to iOS 5; now 80% of music won't play (even in iTunes)

Question

I saw a lot of related questions, but not this issue:  after updating to iOS 5 on my Touch 4g, 64GB.  most of the music simply won't play.  No error messages, the albums and songs still show, but when pushing play, nothing happens.  The capacity bar on the bottom of iTunes shows the data is still there, but for some
reason, most of the music is affected.  Anyone else experience this?  It's the same whether it's just playing on the device or when I plug it into iTunes and try to play it:  won't budge when I push the "Play" button.  How can I "unlock" my music?  (and no, I chose not to get the 'iCloud' thing when updating the iTunes version to 10.5 or whatever the latest one is; I figured I could add iCloud later when I learned more about it).  I also use Manual instead of Sync.

Answer 1

Altered from original solution from Emily P:
Okay, mine now works here is what I did.
plug in the iPhone, iPad etc
in iTunes select the phone or iPad
Under Summary uncheck "Sync with this....."
Under "Music" uncheck the "Sync Music"
Sync
Let it remove all the songs, dont worry it will come back no problems.
Once that is all done all the music on the phone or ipad should be gone.
Now check them both back on, sync, and everything should copy back onto it.
Working now !!!!!