Problem: You've just installed a new piece of hardware and it doesn't work.
Solution: The device may not be seated properly. Make sure cards are fully inserted in the slots. If you are installing an IC into a socket, make sure all the pins are in place and haven't been bent.
Solution: Expansion cards will commonly use a mechanism called an interrupt to control certain functions. Problems will arise if two cards are trying to use the same interrupts. Your cards instructions should tell you how to set the interrupts for that card. Make sure they don't conflict with another card.
Problem: The system clock is wrong or the machine loses CMOS settings.
Solution: The battery in the system may be bad. These can usually be bought at most computer stores and are easily replaced.