If your pressure washer turns on, but won't pressurize water, you don't necessarily have to throw it away.
If it still starts up, but it doesn't spray pressurized water, it's likely just the pump that's gone bad. A replacement pump can be a much more affordable alternative to replacing the entire pressure washer. Most entry-level pressure washers are equipped with wobble pumps, which are not replaceable.
In those cases, you're better off buying a whole new unit; but high-quality gas and electric units have axial cam or triplex plunger pumps, which can be replaced.
First, you'll need to make sure the replacement pump you're buying will be compatible with your current machine. To do this, you'll need to know if the existing pressure washer pump is axial or triplex.
To determine this, look at how your pump attaches to the motor. If the driveshaft of the motor points directly at the brass valves, it's an axial cam pump, which means that the driveshaft is directly spinning the cylinders that pressurize the water.
If the driveshaft runs parallel to the brass valves, it's a triplex pump, which means that the driveshaft is powering a crankshaft that in turn moves plunger rods that pressurize the water. Next, determine the shaft diameter-size and determine if it's solid or hollow.
Look at how the flange on your current pump attaches to the motor, and measure the bolt pattern. You want to be sure the new pump will install as-is. Also, determine if your existing motor is horizontal or vertically shafted - meaning, does your pump mount next to the engine (connected to a horizontal driveshaft) or underneath (connected to a vertical driveshaft)?
You'll have to match your old pump's pressure rating (PSI) and gallons per minute (GPM) with those of an available replacement. Remember, more is not always better - getting a new pump that puts out a lot more PSI can either bog down the engine, or be too powerful for your hose's burst rating.
Check out the installation drawings and owner's manuals for the replacement pumps that are similar to your current pump, and compare the measurements to make sure your new pump will fit properly. Typically, all major engine manufacturers produce the mounting flanges with the same measurements and specs. However, it's highly suggested to double-check your measurements before you order; there is a 25% restocking fee on returned pumps.