thanks for the link. What a strange situation - the domain IS mine, I can operate it from the Namecheap panel, what's more - DNS in Whois is MY DNS. Only the contact info is not mine, but Namecheap's. Also "Transfer prohibited" status seems strange, because it is not included in the FAQ on PIR (possible status is: ok, hold, pending delete, pending transfer). Getting a little frustrated with that...
Edited: - I've finally got the issue resolved
The reason appeared to be simple, but not so easy to recognize - it was a kind of a bug at the profile info at Namecheap. While saving the info, for some reason the ZIP code was not saved (although it WAS filled in). And when I changed the info again, the ZIP field by the default was not filled - so info was not complete and in the result it was changed for the registrar's info. Now I noticed it and changed, and my info is back in whois
Uff... Anyway, thanks for all your answers, it helped me anyway to discover, where to search for the problem's source.