All sorts of things. Can't tell without more detail. This question is a bit like ringing up a car mechanic and saying "my car is broken" and not telling them where you are, whether it starts, whether you've dumped it in a ditch, or even if you're in the same country as the car...
What do you actually get when you try to log in? Screens? What version are you running? Browser? What else have you tried? As Jobin says, most importantly what do the logs say?
Clearing the cache has allowed me to login again, but why? What could be cached on a brand new install so as to prevent me from logging in?
Difficult to say without more information. Did you have an old version installed (then uninstalled, to make way for the new one)? That could do it.
The logs are whereever you've chosen to put them. Look for catalina.out in the logs directory of your installation (assuming you're using Tomcat) and atlassian-jira.log under the install as well.
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