Why does ethernet adapter keep saying network cable unplugged?

Was this Jack working before?  If you plug the laptop directly into the switch does it work?  Would need a cable tester for the wall, something that would test the pins for a line might be crossed or not punched properly which relates to my 1st question.

Does the laptop work at a known working jack?

Does a known working laptop work at the problem jack?

To add to what nick said, have you checked the bios on the laptop and verified that the built in Ethernet is enabled.  Also check the driver.

I still have no clue what happened, but now it's working.  It does that frequently though and I don't know why.  All I did just now was plug a different laptop into that faulty jack and it worked fine on the other laptop.  Then I plugged the cable back into the computer that belongs there, and by some miracle, it worked.  Why does this weird thing keep happening?

It could be a number of reasons:

let's start with a a couple question.

Is this intermittent (meaning you don't change ports, but the nic red X's out periodically)?

..Or does it X out on specific ports?..

Try disabling the wireless adapter and see if the NIC keeps giving you that message.

I never saw a specific OS mentioned, so it's about impossible to give specific instructions on how to disable the wireless adapter.

See if the NIC has any power save options. Some power profiles will disable the NIC if they are on battery.

If it's intermittent, then it might be an issue involving autonegotiating speed with the switch. But also make sure the BIOS and NIC drivers are up to date. You could also try making sure the firmware of the switch itself is up to date, or even testing using a different switch.

Have you tested the cabling with a proper cable tester, instead of just toning it?  It may be that one of the orange or green wires in either pair is broken or intermittent somewhere between the outlet and the panel.  That would give the effect that on the switch the port is up, but at the NIC it looks down.

What switch do you have?  Maybe you can do a TDR test from it?

Thanks for all of your answers everyone.  To answer a few people back....

ChiefIT - I'm not sure what you mean when you ask if it red x's out on specific ports... Like actual wall jacks?.... But what I can tell you is that the little computer ethernet icon next to the clock red x's periodically.

Darr247 - I tried disabling the wireless adapter.  Still gave me the red x.

Craigbeck - We have two switches.  One of them is a Rosewill RGS-1024 and the other is Cisco SLM2024.  It doesn't matter which switch that port is patched into... it keeps giving red x's periodically.  

No, I haven't ran a cable check with a tester yet because I don't have one.  I recently ordered one and I'm waiting for it to ship to me.

The laptops are all running Windows 7 Pro.

"computer ethernet icon next to the clock red x's periodically"

-Intermittent, so it's not a solid red-x?

-Assuming you are using the same switch port and wiring.

This still could be a couple issues.
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The most likely is a duplex mismatch: There is a quirk with cisco routers and switch equipment. The duplex settings have to exactly match the other routers and switches. You can not configure switch to switch or switch to router ports differently. (I say cisco, because it's the only manufacturer I have seen this on).

As an example: Your router to switch ports can not be configured as 1000Mb/Full duplex on the router, while the Cisco switch is configured Auto negotiate the duplex settings. Another example: your Rosewill Switch can not be configured 1000Mb/full duplex while the Cisco is configured to Auto negotiate.
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It still could be other issues, but the duplex mismatch is something I have seen most commonly causing this problem.

Try setting your NIC and its respective switch port to 100 mbps. If the drops end, them it was indeed a negotiation issue. HP can be dumb in this regard too sometimes.

If it is a negotiation issue, that usually points back to cabling.