Cannot access specific website (MBP - OS X 10.10.5)
1 - Run Disk Utility from the Utilities folder and "Repair Permissions."  Let it run.  It will take a while.  When it is done, quit the program.

2 - Reinstall an anti-virus program and run a scan.

3 - Open Safari and from the "Safari" menu, click on reset Safari.

4 - Go to System Preferences, click on "Security and Privacy."  Make sure your firewall is set to "ON."