Do you know what the issue is?
Can they still ping?
If they can ping by IP but not by DNS name then there is a DNS issue.
If they cannot ping the server then can they ping anything?
I'd check the logs both on the PC and on the server - although I suspect the
problem lies with the PC. I'd also check to see whether a more recent driver
is available, as an updated network card driver can cure various problems.
I have known the power saving feature on laptops disable a network adapter, so
that is something else to consider.
Failing that, it might be a hardware problem.
Let us know how you get on.