Play it Safe
Unplug the opener. That way, you won't lose a finger if your unsuspecting housemate hits the remote button while you're working. Even worse, you could electrocute yourself, in which case you wouldn't be able to blame your housemate at all.
Symptom: The Remote Works But the Wall Switch Doesn't
If the remote works but the wall switch doesn't, you may need to replace either the wall switch or the switch wires. To determine whether the switch or the wires are bad, first unscrew the switch from the wall and touch the two wires together (don't worry, the wires are low voltage and won't shock you). If the opener runs, you have a bad switch. If you have an older-model opener, a cheap doorbell button might work. If you have a newer opener that has a light and a locking option on the switch, buy the one designed for your model. A new one should cost you about $15.
If the opener doesn't run when you touch the wires at the opener, use a small wire and jump those same two wires at the opener terminal. If the opener runs, the wire that connects the opener to the switch is bad. Sometimes the staples that hold the wire to the wall pinch the wire, causing a short. Install 18- to 22-gauge wire as part of the garage door repair.
Symptom: The Wall Switch Works But the Remote Doesn't
Fix: Replace batteries or buy a new remote or receiver
If the wall switch works but one of the remotes doesn't, check the batteries first. Still nothing? You may need a new remote. Home centers carry a few models, and you can find a wide selection online.
If you can't find one for your garage door opener model, you can try a universal remote or you can install a new receiver. A receiver replaces the radio frequency the opener uses with its own. An added bonus of a new receiver is that it will automatically update older openers to the new rolling code technology, which stops the bad guys from stealing your code. Just plug the new receiver into an outlet close to the opener, and run the two wires provided to the same terminals the wall switch is connected to.