Regarding getting your music back... if you didn't make a backup before deleting the files there's a couple of things you can do. One is to re-download them from the cloud. You can highlight more than one track at at time, right-click and select "download." Or, any music you ripped from CD you could re-rip.
Whatever you decide to do you should then copy the music to an external HDD and have a backup scheme in place for when you add new music to the iTunes library.
Backup, backup, backup. If you do not backup your personal data you will lose it someday.