Hi,
I think you copied the "Users" folder into your external hard disk. So first we can restore the iTunes.

So copy the complete iTunes library from ~/Users/[your name]/Musics (From your external hard disk). Copy the whole folder "iTunes" into your internal hard disk at the same place i.e ~/Users/[your name]/Musics. (Warning : This will replace the entire iTunes library in your internal hard drive of your iMac. So make the backup of the iTunes library of your internal hard drive's itunes library into any other place to prevent the loss of data. )

Now launch iTunes and see if you are able to see all the contents of your iTunes.

Please let me know if this works or not.

Only Click ACCEPT after your problem solved.

Glad to hear that iTunes worked...Now we can look into iCal, iCal is not very easy as iTunes. First, quit iCal if it is opened. Then to goto ~/Users/[your name]/Library (in you external hard disk) and you can see a folder named Calenders. Copy the folder and paste into the same location into your internal hard drive i.e into ~/Users/[yourname]/Library

(Warning : This will replace the entire iCal entries in your internal hard drive of your iMac. So make the backup of the iCal from your internal hard drive's into any other place to prevent the loss of data. )

Then goto ~/Users/[your name]/Library/ and open the folder named "Preferences". You can see a file named "com.apple.ical.plist". Move it into desktop and restart your Mac. Now open iCal see if the entries are there.

Please let me know if this works or not.