What's version of MySQL? 5.7 or higher?
Fresh install of MySQL 5.7+-> is now different than previous versions. After fresh install there is a temporary root password for DB server. Did you see a notice about what/where that temporary root password was located?
Did you run mysql_secure_installation right after install?
Using the root credentials you think you know ... assuming mysql server installed on localhost and you have installed the mysql client, try:
mysql -u root -p'passwordyouknow'
Does the client connect to the server?