I have a minor but niggly problem with a HP Pavilion s3734uk. Ever since the BIOS battery went flat and was replaced, when exiting sleep mode the LED in the power button stays yellow and there is no output from the monitor. The problem is easily cleared by disconnecting the mains lead and allowing it to discharge, but it's annoying not having sleep mode available. This happened from the time when the battery first went flat - Before I replaced it, so I don't think I unseated anything when I replaced the battery, but it has been the same ever since.
Logic suggests a defaulted BIOS setting is the problem, but there are very few changeable options on this model, and none seem to pertain to sleep. There isn't even an S1/S3 option. Hibernate is not supported.