Hi there Pikex,
Sorry for the inconvenience of not being able to join in. That error usually means that Mojang's authentication servers are down meaning you and a lot of other people are not being allowed to join whilst Mojang fixes the issue. You can check the status of their servers here :
Mojang Status
As this has been happening for a couple of days, it may be that your anti-virus, or network settings are preventing Minecraft from accessing the Mojang servers. I can take you through some troubleshooting steps also if you need further assistance, please reply here if you are still in need of assistance.
Thanks!