Hi Steve,
so where were the 2500 songs originally stored? on the pc itself or always on the cloud?
if they were on the pc, you can use software to recover them from the hard drive even if they've mysteriously disappeared
if they were on the iCloud and disappeared that's a slightly different situation
As far as I remember , music files are stored locally on the PC. If you synced the new device to iTunes it will have just shown you what's on the new device and unless "auto sync" with the music library was selected, it won't have transferred your music automatically.
The other thing to check is if the option for iCloud Music Library is turned on for both devices.
This website may / may not help
http://http://m.imore.com/everything...-music-library
Hope that hasn't confused you even further! Apples insistence on using iTunes and locking down their devices has really made it hard to troubleshoot problems because the workarounds are rarely simple.
also...if you can help it, I would advise very strongly moving away from iCloud completely. It's a broken system and isn't the seamless utopia that Apple make it out to be. I had so many issues with it I dropped it like a bad habit.
Dropbox is a better alternative or something like Synology's NAS drives