The problem has finally been solved - I suddenly remembered a place that installs and repairs car radios, among other things. I went there, they removed a chip (which I didn't know existed, in a little box under the dashboard labeled "map" and apparently meaning GPS), put in a different Hyundai i30 chip, fiddled with the settings which had reverted to default, reset them, removed the chip they had put in, put back the old chip and sent me on my way. It cost me a little over $40. However they set the time wrong and I had to take it back to get it reset - this time they didn't touch the chip but just played with the reset buttons until it finally worked (it took half an hour).
Thank you to all who tried to help.