I finally got a hold of a Dell technician who was able to understand my problem. I was told to do a power cycle of the laptop: Remove the battery, remove the memory sticks and finally press the power button for a good 30 seconds. Then wait another 30 seconds before you put everything back together.
In another words, just a full CMOS reset. But it worked! After booting the TPM options were back in "Security" in the BIOS.
I still would really have liked to know what caused this in the first place though.