added ram and now cp won’t turn on

So I decided to add 512mb ram to my hp pavilion
523n desktop which previously had a 256mb and
512mb sticks in it in order to help speed
things up but now that I added the new 512 with
the previous 512 RAM it won’t start up at all.
I tried going back to the previous RAM and that
won’t work either. Any ideas? Help please.