Motherboard screw stuck in standout
When removing a motherboard from a computer case, it is necessary to remove all screws securing the motherboard inside the case. Generally, standoffs are located between the motherboard and case. These standoffs act as spacers between the motherboard and case, to prevent the motherboard from shorting out by touching the metal in the solder points to the metal of the case.
Sometimes when removing a screw from the motherboard and standoff, the standoff will unscrew from the case instead of the screw from the standoff. If this occurs, it will be necessary to unscrew the standoff separately.
To do this, using a pair of pliers (needle-nose pliers will probably be the easiest to use), hold the standoff in place and with a screwdriver, unscrew the motherboard screw. The screw will be separated from the standoff.
Most cases give you access to the underside of the motherboard by removing the opposite side of the case. Hold on the standoff on this side of the case.
If the motherboard screw is difficult to remove from the standoff, even when using pliers, do not use grease or oil to attempt to loosen the screw. Grease and oil could cause damage to the motherboard. Also, the screw should always be snugly screwed into the standoff, never try tightening the screw in more than a snug fit in hopes of screwing the standout more into the case. Doing this can damage or break the motherboard.