Go to your apex and do the following:
System > Net setup > IP Address > 'setup' and ensure your IP address is the same as what you programmed it to be in your apex.
If it's not, obviously fix it to be the same IP address as your computer.
If it is and you still can't access the apex program by typing
http://apex into your browser then reboot the router and your computer and it should work.
NOTE: On a mac you can't enter
http://apex to access the program, you need to enter in your IP address which would be something like 192.168.0.XXX in to your browser.