Q: I haven't used itunes for sometime and now have a new PC.  I have updated my account (changed password) but when I log on all my music has gone?

Unless you've subscribed to iTunes Match (which isn't a complete solution anyway) your iTunes Library is on your previous PC - simplest solution is to copy it from there or from a backup.  See How to move your iTunes library to a new computer - Apple Support for more information.  Previous purchases from the iTunes Store may be available for re-download - see Download your past purchases - Apple Support.

If neither of these approaches allows you to recover your library content, see turingtest2's user tip on Recover your iTunes library from your iPod or iOS device for a list of tools and methods that can be used to copy media etc. from your iPod to iTunes.

Have you put your music on your new computer?

Copy everything from your old computer, or your backup copy of your old computer to your new one.