Check your server logs, there should be some detail that gives a clue. Also, Cake has its own logs under tmp/logs which might help. Without any such details, there’s simply no end to the guesses we might make about what’s gone wrong.
Server logs will be wherever your server puts its logs. Cake’s logs should be in a folder called tmp/logs under the Cake installation. Shouldn’t be hard to find. You should be able to turn on more debugging by setting “debug” to 2 instead of 0 in config/core.php.