It has nothing to do with what browser you are using or anything else client side, this is all server side.
- Someone has either messed around with the server files and changed the connection credentials to connect to the mysql database.
or,
- someone has accidentally deleted everything to do with mysql on the server,
or,
- the server has been updated to php 7 --- those msql commands were deprecated back in php version 5.5 and as of php 7 they have been completely deleted, so any websites using those commands stop working when upgraded to php 7.