Eugene's observations

In normal computer operation one or more 'dirty bits' are set; these are on/off flags to show that the computer is running. As part of a normal shutdown, these bits are unset. Next time the PC starts the bit(s) are tested and if set, a problem in shutting down previously is assumed and an automated repair operation is initiated in an attempt to resolve the problem without intervention from the user.

While the repair is taking place it may not be possible to display anything on the monitor screen and so it is easy to assume that the computer has stalled in startup and that it is idle. It probably is not. Sometimes the 'Welcome' screen is shown seemingly for ever. In such situations step away from your computer.

If you assume that it is not doing anything (i.e. it has hung) and power down, you will probably be causing even more damage necessitating a call to a repair technician. Better to listen for hard disk drive activity and look for a flashing LED to see if the computer is doing anything. If it is, leave alone for 15 minutes and then look and listen again to see if the hard disk drive activity has died down.

This is especially true for recent operating systems (e.g. Windows Vista) as more checks and automated repair procedures are built in. Some consequences of an abrupt power down are listed on this blog at 'Abrupt power loss - the effects of hitting the big red switch'.