Can you give us more info on the type of connection you have (modem dialup, ADSL etc) and what equipment you have?
Basically your modem cannot establich a PPP connection to the ISP - but I guess you know that with not being able to connect.
PPP is the protocol used to establish your connection, login to the ISP, receive your IP address and various other params to control the transmission of data.
No idea why it does not work. Are there any other logs around that one line?
Can you try connecting to the master socket if you are currently using an extension socket or cable. Try removing all other phones in the house?
Can you make ordinary phone calls on the line - i.e. it is working?
You may need to raise a contact us ticket to get PlusNet to check your line. It is possible there is a fault and your modem cannot estabish a connection. The current weather conditions may be playing a part. - is it snowing or raining?