This same problem happened to me, plus I have some additional info.
Just like the earlier poster, I am a longtime user of Windows Live Mail. I was prompted to update and did so, and liked a number of improvements.
However, the email addresses of my contacts had been removed, along with all info other than just the contact names. The number of contact names is unchanged, and as far as I can tell, all the same names are there. The few email addresses that do remain (about 20 out of 500) were all used recently, AFTER the first launch of the previous WLM version on this computer, which is new since February. My guess is that these email addresses were automatically added per the preference setting "add email addresses to contacts after third reply."
Here is the added info: when I tried to import my contacts again from a CSV file, it didn't work. A window came up saying that the program was updating my contacts, and it took a REMARKABLY long time, but after it finished, I was left with the same scenario. My CSV file is still good, but WLM will not work with it.
I'm out of ideas. Please help.
Thanks to all in advance.