Lost music

Is the view you are using the one that shows music on the iPhone? Like the one shown below:

Note that the view is On My Device/Music and in this view the greyed out songs are waiting to be added. However, there is an issue at the moment whereby iTunes fails to synchronise all the songs it is supposed to do at a Sync. The solution, until Apple fix the issue, is to Sync again, preferably without disconnecting the phone from iTunes.

... or are you looking at the (same) music in iTunes?

First of all you should work out why the music has been lost, otherwise it may well disappear again.

• What update are you referring to, iTunes, Windows or something else? If it was iTunes, which version are you now using?
• Is it all of your music that is missing, or just some of it?
• How do you know it's missing - that is, do you see any error messages or an explanation mark to the left of a missing song's title?

Once we have those answers, we can either find the missing music or suggest some ways of replacing it. Finding it would be the easier option, I'm sure.