All apps installed on my SD card have suddenly disappeared
As Manishearth wrote, this might indicate trouble with the card (and very likely does). The boot you describe might have been "too abrupt", not cleanly unmounting the card and thus corrupting its file system; in which case the "good ol' chkdsk" should help (btw, Android ships with fsck for the very same task, which you could execute via a terminal app or adb shell – see Check and fix SD card errors within Android itself?).

On the other hand, it could also mean hardware trouble with the card, which even could be the reason for those reboots, reversing cause and effect: Not being able to read/write, or encountering too many "corrupt blocks", might have triggered some event (like re-mounting the card read/only). I'm not sure whether such would trigger a reboot (as stuff counted essential by the system usually doesn't reside on the sdcard), but it wouldn't be impossible. Thus trying a new card won't hurt.



PS – A little background on app2sd: As shipped with Android 2.2+, this only moves parts of the app to your card, while other parts remain on internal storage. Especially the app's data remain here (in /data), which is why after a re-installation of your apps they were already (or rather "still") available. For more details, you might wish to take a look at our app2sd tag-wiki.