Unfortunately, we can't tell you why someone blocked you, but maybe you can take a look at the rules of conduct and find out why the member blocked you.
Internalize them so you won't be blocked again ;-)
Maybe it's also a moment to reflect on your cover letter. Sometimes a written message comes across as more unfriendly than you might have intended. Or maybe you were a bit too hasty?
Just take a look at your sent messages.