Sometimes a bin does not heat up and, when broken open, some or all of the contents remain relatively unchanged. Given time they would undoubtedly decay, but we are necessarily impatient. The answer is usually to fork out the materials onto a plastic sheet, breaking them up into smaller pieces as you go. If they are dry, wet them; if wet, add dry stuff. Then sprinkle on activators and/or fresh green material and pack it all back in the bin again. Ninety-nine times out of a hundred the pile will heat up, cook and compost. Just possibly it may still fail, but only if lots of something really hostile was included, say, too many evergreen leaves or wet manure soaked with disinfectant, or similar.
If too wet
As said above, remix the material, adding shredded paper, cardboard, straw, hay, leaves, sawdust or other dryish stuff. Too-wet conditions will also probably have leached out nitrogen, so add some of the manures or activators listed above or a nitrogenous fertiliser. Be careful to mix thoroughly and do not pack down but leave contents as loosely aerated as possible. Add plenty of lime and/or soil to the topmost layers to absorb ammonia and the gases that wet bins produce.
If too dry 
Again, remix and while the contents are spread out on a plastic sheet wet them down with dirty water. Add a drop or two of washing up liquid to better wet really dry stuff. Pack the moistened materials firmly back in the bin and rewet again as you go, if necessary. A bit more nitrogenous waste will again help.
If too acid 
This is often associated with too wet. Usually in the absence of lime or wood ashes the materials may rot but remain recognisable, and probably whiff a bit. Remix them incorporating some lime, wood ashes and or soil, and to promote heating mix in copious fresh green materials and/or manure. Remember, this compost cannot then be used on ericaceous lime haters.
If too alkaline 
An unlikely eventuality, as most compost becomes slightly acidic. However, if you accidentally included too much lime or wood ashes (or monumental quantities of marble chips, eggshells, snail shells or bone ashes) the best corrective would be to remix with huge quantities of grass clippings and leaves.
Bob's Basics: Composting
Composting is one of six indispensable gardening companions from leading organic gardener Bob Flowerdew, for both the gardening novice and experienced alike, to guide you through the seasons. Composting is available at a discount from Saga Books.