Can't connect to internet - please help

What's the full text of the "ISP failed" message?

Confirm the network settings are correct; compare the settings and responses from the DHCP server (or the static settings) are correct for the particular network; IP address, IP subnet mask, gateway router, and DNS server(s).

If you're using a wireless connection (as can be inferred), are you in an area where you're likely to encounter interference from other WiFi devices or other devices (cordless phones, other wireless devices, etc) transmitting in the same band you're using for WiFi? Or walls or other structures which might block the wireless signals?

If you are using the wireless connection, switch to the wired connection and see if that works.

If you are using the wireless connection, confirm the credentials, and move to a different location (and preferably closer) for the WiFi router.

If you're using a wired connection, switch to a wireless connection.

And confirm that any DHCP server that's in use here is correctly configured; that it's not passing out IP addresses that are incorrect for the subnet (if you're using more than one DHCP server), or addresses that overlap with any devices configured with static IP addresses.

If you're running more than one DHCP server or more than one WiFi router, consider consolidating to one DHCP server device, and running the wireless devices as access points; what Apple refers to as "bridge" mode with its Airport and Time Capsule devices.

The documentation for that Network Diagnostics tool is certainly lacking. Since I'm not showing an ISP error here, I don't know what the text in that tool states for that error, and (given I use the command-line tools for this stuff) can't see what it might be complaining about. (What's the text in that tool claiming that ISP error means?)

I'd tend to expect that ISP red-light means there's an issue connecting to the ISP-specified DNS server(s), but that's a guess.

Launch Terminal.app (in the Applications > Utilities folder) and issue the following two commands:

$ dig +short -x 8.8.8.8

$ dig +short @8.8.8.8 -x 8.8.8.8

You should get some variation of "google-public-dns-a.google.com." as the response for both.

This thread is a reset sequence for the network connection, which might resolve this if something's gone wonky in the saved settings. Remove the settings and passwords, and re-add them.

I don't know how experienced you are with troubleshooting this stuff and (without posted data) I can't see what the configuration is showing here; what you've looked at here, and what you've already tested. Given you're not posting these details (and for whatever reason), you might want to ring up Apple support or your preferred OS X support provider and have them work through this with you.