The strike plate receives the door latch and/or deadbolt for your door lock. If they don't line up the lock won't work properly.
If this is not your problem, see 'Door Repair' for all of the Door Repair topics.
Is the Door Adjusted Properly
Many problems with doors and hardware can be solved with some adjustment to the door. Assuming the carpenter installed the door correctly originally, what would cause it not to work now? Age and gravity are the two main things that will affect a door.
Over time, the weight of the door will cause it to sag. When that happens the door strike will not align with the strike plate. Tightening the hinges or adding a support screw may be all that is need to get the door working correctly.
Moving the strike plate should not be needed most of the time. Before you attempt to adjust the strike plate, make sure the door does not need to be adjusted. See 'Adjusting Interior Doors' and 'How To Adjust Exterior Doors' for more information and instructions on how to align the door.
If the door is adjusted properly, there would be two reasons to move the plate. One, the lock is not working right. Two, the door is not tight against the weatherstripping.
Moving a Strike Plate
The strike plate may need to move in or out slightly to allow the strike to seat correctly. Move it away from the center if the strike will not seat when the door is closed.
For an exterior door you want the door to snug up against the weatherstripping. To snug the door up you would move the strike plate toward the center of the door jamb. Move it away from the center if the strike will not seat when the door is closed.
For minor adjustments, you may be able to loosen the plate and tap it in the direction you need.
Use a screwdriver to loosen the screws slightly. Use a block of wood and a hammer to tap the strike plate in the right direction. Hold it in place and tighten the screws.
Chiseling the Mortise
Moving the strike plate any significant amount will involve removing it and chiseling out the mortise. Use the screwdriver and remove the strike plate.
With a sharp pencil draw an outline of where it needs to be on the jamb. With a sharp chisel you can trim out the wood up to the line. Moving the plate slightly will give you a problem with the screw holes.
You might need to whittle a little filler pin and glue it in place. This will give the screws a place to grab in their new location. Otherwise they will want to go back into the old location.
When the glue dries you can trim the pins off with the chisel and install the strike plate. Depending on which way you moved it, you may need to trim the mortise, the one where the strike seats. With the strike plate properly located the door lock should work.
Did you need to adjust your strike plate, or was it the door out of adjustment? Moving or adjusting a strike plate is not that difficult, hopefully you were able to solve the problem.