So since the weather has started to cool I've noticed a surprising number of these threads. I'd like to propose that we make a thread with some basic information (that could maybe be made a sticky) which can be the first point of call for anyone before they make a new thread.
I'll start. Please feel free to add any info you think is relevant.
First thing is to stop worrying. Snakes will go for months without eating in the wild and even a young snake can go for quite a long time without issues.
Have you just obtained the animal? For most inexperienced keepers, new snakes should be given a minimum of two weeks to settle into their new home before being offered food. Give it time to get used to it's surroundings and become comfortable.
Is the animal in shed? Lots of people have snakes that feed during the shed cycle with no issues but some just won't take any during this time.
Next thing (and probably the biggest) is checking temperatures. What is your hot end temp? Anything less than 27-28 is too low for most snakes and won't provide them with the right conditions to digest their food. Even if you are getting a reasonable hot end temp it may not be staying that warm for long enough (in the case of snakes housed in garages and sheds, ambient temps can play havoc with this).
What time of year is it? Many snakes can sense environmental changes (not only temp) that signal the cool season is coming. It seems this is most prevalent in Antaresia species. In this case, you may have perfect temps but just have a snake that doesn't want to eat. For anything over about a year old this isn't a big issue. You can continue offering food on a weekly/fortnightly basis or you can just lower your enclosure temp and let them brumate until spring.
An important note is that if your snake refuses a feed, don't bother offering another item the next day. Leave it at least a week. If stress is a reason for it not eating then all you are going to do is stress it more by sticking a rat or mouse in it's face every day.
Lastly - STOP WORRYING. Unless you have a very unhealthy snake you have very little to be concerned with if it hasn't eaten for a month.
I'm no expert by any means but these are the main issues I've noticed. Like I said please feel free to add anything you feel I've left out. Hopefully this will be useful for some new keepers in the future.
Dan