"Please plug phone line into modem"

I recently unplugged the phone line to my desktop so that I could use it on my laptop and after reinsterting the phone line into the desktop and trying to connect to the internet, the following message pops up:

"Please plug the phone line into your modem"

I've followed various trouble-shooting advice to solve the problem, including checking the line to see that it works (which it does). I'm using the same phone line to connect to the internet via my laptop now. I also thought there might be some problem with the way the internet server was set up, so I reinstalled it and even reconfigured the modem.

It seems the problem is with the port, but I'm not a technical expert by any means so I'm not entirely sure. What should I do?

Thanks!