Links not working

I installed 2.7 Beta1 but clicking the index page brings nothing but the same page (the root directory). Admin side perfectly works. May be it’s a bug so I tried the last stable version (2.6.3). This time i was able to see my front page and all that except one thing, clicking any hyperlink (comments or links on the right side) brings the root directory index. Here again, admin side works perfectly. Help please.