Probably one of the following:
1. Wrong server name/user name/ password.
2. Comunication error:
If you are trying to access a remote machine there might be comunication failure, server might be down, etc. Try to restart the machine (Where MySQL installed) and try again.
If you are having a problem in the same machine MySQL installed it can be either the server is down or firewall is blocking the access. Reconfiguring the firewall and restaring the server should solve it.
Good Luck, Gargamel.