Help Multiple problems after changing SD cards

[wookiedroid]

I put an SD card from an HTC Desire in my Galaxy S, browsed the files on the SD, and viewed some of the photos. I then replaced my own SD card, and on booting the phone it started acting like a new phone, e.g. it showed me the Orange Homescreen selector on boot, all my calendar entries are gone, all my phone contacts are gone (but not those on the sim). Most apps I have installed recently (possible since I first installed the SD card) crash unexpectedly, unless I uninstall them and reinstall them. Even after reinstalling apps, some of them repeatedly act like they are being run for the first time, e.g. they show an info screen with 'Don't show this again' unchecked even though I have checked it before.

Any ideas what might have happened here and how I can fix this problem with the apps? Any chance of retrieving my phone contacts? I don't have a backup.