Try closing down and re-starting the computer.  That solves lots of problems.

Try adjusting the brightness knob if screen is black.

Is the green power light on? (It should be.)

Is the computer plugged in? Are all cords connected securely? 

Does the electrical wall outlet it's plugged into work?  Plug in something else there to test it.

If it's plugged in to a power strip, is the power strip's switch turned on?

If having trouble accessing a web site, double-check your spelling.

If having trouble saving, try a new disk -- they do wear out after a while.

SAVE YOUR WORK FREQUENTLY.  If you are working on something really important, save to a disk and to the C: drive of your computer.  That way you'll have a back up in an emergency.