Sim contacts seem to have been overwritten by sync

Contacts aren't stored on sd (you can "export" a backup to sd, but that is just a backup rather than a working database), so it must have been the SIM. Does the contacts app not have an option to show contacts on the SIM, or does it have the option but doesn't show any? I have no contacts on my SIM but still have the option to show that category (different phone, different contacts app), so my guess is that if it doesn't even have the option it's more likely that the phone software doesn't support it than that they've been deleted. Have you had a system update recently? That would be one way the software could change.

One thought: does the Contacts app have the option to import contacts from the SIM? It's under "manage contacts" on my phone - the same menu can be used to import contacts previously backed up to sd. If your contacts are on the SIM (or backed up to sd) but not visible to the Contacts app then that may let you reload them.

And a stupid question: you have tried just rebooting the phone?

If you can get them back then it it's important to back them up straight away, and whether that's by exporting to sd and copying to a computer or syncing with some cloud service you want a backup that's not on the device. SIMs and SD cards can both fail, and devices can be lost or stolen, so keeping your only copy of anything important on the device is always risky (we see so many people who lose baby photos or last messages from dying parents because they didn't think of these possibilities until they happened).