This occurs semi-regularly on MT4 as the program cannot process multiple requests simultaneously. The easiest solution is to close your MT4 platform and then open it up again.
The ACY demo platform is valid for 30 days, so check to see if your demo platform is still valid. To generate a new demo account, go to File > Open Account, and you can create a new password and login. You won't need to download another MT4 platform – you can use your existing one but be sure to make a note of your new login details.