Everytime I access the site, that error will be shown,

Database error: cannot connect to Database
MySQL Error
Session halted.

Anyone can help me with this error?
Super thanks.

the error message is telling you the application cannot connect to the database
that could be down to anyone of a number of reasons. the most likely is going to be an incorrect userid + password or insufficient permissions for that userid for the requested actions

1) the db isn't active ... check the server is running
..first off check the server is running on the computer that MySQL is installed, what you are seeking to prove is the server is active and able to receive connection requests. how you do that depends on what machine the server is running on, what access you have to it and so on.
..asssuming the server is active
2) check you can talk to the server from another application (try downloading 3rd party software such as MySQL Workbench, or anyone of the many other tools such as HeidiSQL, PHPMyAdmin, modelling tools and the like) and see if you can connect to the server of the options Id strongly recommend you use workbench as it helps int he administration of the server. a problem here could be a firewall blocking connections for / from MySQL
3) check your code, make certain the userid & password are correct, make certain the host is correctly sepcified. investigate any error logs to see if they give a clue as to whats (not) happening

you don't give an indication of what language you are suing to connect to MySQL.

Thanks for the reply.

I'm using PHP to connect to MySQL.

I'm not really good on databases.
Hope I can get all of what you've said.
THANKS SO MUCH.

show us the PHP code that is falling over, but only the code that is failign we don't need to see the whole php script, just the bit that opens the the connection, selectd the db and so on
do you use error trapping?
do you use the "or die" construct?php or die - Google Search

at present I'll leave the post here, but its more likely this will be a PHP problem rather than a MySQL problem, although its probably down to the user permissions