Well, my 1990 300TE did exactly the same thing about 3 months after I bought it with one key and 125K miles 2 years ago. For no apparent reason the key wouldn't work. Went to the dealer with the title and the key itself. The dealer had a brand new key sent sent to them (they couldn't actually make a copy themselves!). The new key worked great right off the bat and has ever since, and I swear, I hold this new one that
works side by side with the old one and I cannot see any difference at all, they look identical, but the new one works and old one won't...still....I try it from time to time....the ignition key is an interesting little monkey....