If you search on Google for the term "joomla can't login after moving to new server" - or something similar, you will get more, than 1 million hits for most of the combinations.
So, you aren't alone. And there are plenty of suggestions, on what might gone wrong and how you might find that, and there are verious fixes. But what if you moved the site to another domain? From development server to it's final place? This is a subject wich is rarely touched by those advices.