There can be many reasons why a wordpress login fails. Most of the time the problem is caused by an out of date or broken/poorly coded theme or plugin.
The easiest way to test if your theme is the cause would be to connect via FTP or cpanel file manager and rename the folder of your active theme. Then refresh the login page and try again.
If that does not work then your next best step is to rename your plugins folder to anything else (I use plugins-old), then refresh the login page and attempt to login again.
If you are still experiencing issues then the cause could be due to a hacker or problems with the server. That is when you would want to open a support ticket and request assistance. Please be sure to provide any errors (screenshots preferred) and your wordpress login info so we can test.
