The scenario: Your Internet connection doesn't work, leaving you stranded from the outside world--or at least, from cat videos on YouTube.
If some of the computers on your network can access the Internet, but not others, you're dealing with an issue on your network and the connection to your ISP is working fine.
Unless your ISP supplied both the DSL/cable modem and the router (wired or wireless) that you use to share your Internet connection across the network, it is not likely to provide much helpful tech support besides telling you to power-cycle (turn off and on) your modem and router and reset them to factory settings. You're better off checking out our "Home Networking Guide" for help.
On the other hand, if you're completely unable to access the Internet at all--even if you directly connect a PC to the modem via ethernet--then it's pretty clear there's a problem going on with your ISP, not your own equipment. Try power-cycling everything and checking the cable connections, just in case.
Prepare for your call: First, try calling the ISP's main support line--if there's a large service outage in your neighborhood, there will often be a prerecorded message telling you what's going on and saving you the trouble.
If there are no local service issues, you'll need to gather some info before making your call. Get the account holder's information--that's usually your account number (if you have DSL, it's probably your landline telephone number) and the account holder's social security number. This can make the call complicated if you're living with roommates (or if the account holder passed away two years ago, as was the case in the last apartment one of us had), which is why you need to make sure you have all the information before setting aside time to call. Also, if it's a speed issue, know what level of service you signed up for.
You should also have information about your hardware handy. Know your brand and model number for your modem and/or router, as well as the MAC address (a twelve-character long string of numbers and letters, usually punctuated by colons or dashes, that can usually be found on a label on the device). You'll also want the MAC address of the Wi-Fi or Ethernet card that you use to connect to the Internet, which you can find by opening up the command prompt (type cmd in the search bar) and type in ipconfig /all to bring up your networking information. Scroll up until you find your device (Intel WiFi Link 5100, for example) and write down the Physical Address.
During the call: Start by getting a number to call back and a case number in case you get disconnected.
Calling an ISP's tech support for an Internet problem can be tricky because if the tech has any reason to believe that the issue might be with something other than the Internet connection itself, they might just tell you to call a different tech support line. For example, if you're using a wireless router that didn't come from your ISP, the tech might very well tell you to call the router company instead, or your PC manufacturer's tech support line.
Avoid this by keeping your network as simple as possible (directly connecting to the modem via Ethernet, or setting up your network with your ISP-supplied router) and if you can, make sure that your computer can still connect to the Internet through other means (your local cafe or library Wi-Fi, for example). This way you can clearly establish that the problem isn't your computer or your network, but the ISP connection.
Home service call? Internet support calls tend to be closer to calling your utilities company than fixing a PC over the phone; you call them when you're not getting Internet coming down your pipes, they give you a few quick tests, and then they send someone out to fix the problem.
Usually the issue appears in at least one of three spots along the line: your service hasn't been enabled on the ISP's end (happens most often with new customers), something isn't connected at the switching station between your residence or office and the ISP, or something isn't connected in the wiring on your end.
If the ISP simply hasn't enabled your service yet, it should be easy for them to turn it on during the phone call. If your connection is having problems at the switching station or on your end, however, the phone tech will have to send someone out to fix it. Switching station checkups can generally be handled before the day ends, but on-site visits mean you'll need to free up some time to let the tech in your house or office, if necessary.
Hopefully, all goes well with the tech's visit and your Internet access returns--but prepare for the worst. Keep your account info, case number, and callback numbers together in case you need to make another call so your ISP's phone techs know which repairs have already been tried.
Bonus: Keep track of the time you spent waiting for or talking to tech support, any problems you had dealing with staff over the phone, and especially the dates/times of service outages and any fees you were charged. ISPs aren't contract-locked like cellular providers or a one-time purchase like most tech products, so just call up their customer service line with a record of your service problems and you'll probably be able to get a credit to your account plus a free month of service or so.