When a connection to the internet is established, there always exists a modem between the connected device and the ISP (Internet Service Provider). The Operating System (OS) on your PC controls the modem which communicates with the ISP to establish a connection.
When you cannot connect to the internet, the most probable reason is that either your modem is not configured / installed properly or the login parameters being used are incorrect.
You can think of it as a simple phone in your premises which is used to make a Call. We can list the same process as below –
At your premises –
1. You pick up the phone and hear a Dial tone.
2. You dial a Number to the person you are calling.
3. The phone company checks if you have sufficient credit to make the call.
4. If you pass the test at step 3, then you are connected and you can talk to the person on the other end but if it fails then the connection is unsuccessful.
5. When finished, you hang up.
In your PC –
1. The Operating system checks for a modem and initializes it.
2. The Operating system dials in to your ISP.
3. The modem requests a connection to the internet with your ISP which checks your user credentials.
4. If you pass the test at step 3, then the modem establishes a successful connection to the internet and you can browse the web but if it fails then the connection is unsuccessful.
5. When finished, you disconnect.