Or: DQTG. The guide which name takes longer to say than the remedy itself.

• Restart your phone. Do a soft reset, on phones with removable battery you just remove the battery and wait a while. One One X and such: you press power button for 10 seconds (it will count down)
• Having problems with phone after update? Check this
• Your Windows phone do not react at all? Press power button for at least 20 seconds. This works with most android HTC phones as well, even tablets.
• If you’re having problem with calling and sending texts, don’t play around with settings in your phone for it. Remove and check your sim instead. Even: make sure that you’ve got balance on your prepaid card (I’ve seen this). Check the simple little things. Check that your network is ok if you just can’t get online.
• Your One X (and such) died since battery ran out. And now when you put it on charge just nothing happens. Relax: it’s ok, if battery is very flat: it will do a pre-charger routine on the battery and the charging led will not come on at once. It can take up to 20-40 min for it to come on, and when it do it might flash for a while.
• Charging led don’t come on! see above, and if it still doesn’t. Check with other cable and/or other charger. If it still dead: call support.
• Phone is running slower and slower. Start with soft reset, followed by an factory restore if it didn’t do much. And: be a bit restrictive with apps after the factory restore just to find the culprit of the slowdown.
• Reboot into Safe mode just to see if the app you’ve recently installed is to blame for your phone acting up (often is the simple reasons).
• Use a bit of common sense: it’s rarely complex things behind the problems, it’s often the simple ones.

I am sure that there’s more, either by me or you reading this..