In all the years that I have done service, by far the most common problem with washer failure is in the drain system.  Either the valve fails, or there is blockage in the line somewhere.

This is easy to check on an open sump type drain, all you have to do is follow the drain line going into the sump (a little bit of water is normal).

If you have a water valve stuck on chances are it needs a rebuild kit.  Sometimes you can get away with taking it apart and clearing any debris that might be causing it to hang open.  Before ordering a new rebuild kit make sure the seat inside the valve body is not cracked (if it is, then the whole valve will need to be replaced).  Sometimes you can free up a valve just by tapping on it. If that doesn't work you can try turning off the water valve.  (the problem with this is, if it's a hot valve, you will only be able to run cold cycles) some machine have more than one hot, and more than one cold valve. (I've seen housekeeping staff run a garden hose to the hot water heater and fill it manually too.)

Machine won't start (nothing comes on: display or lights )

First step, check that the breaker or disconnect is on (or that the line fuses are OK - even though line fuses should'nt be used. . . ) If that's ok, check for power where it goes into the machine (j-box) you'll have to use a volt-meter for that one.

If power going into the machine is there, check the control fuses (these are the glass type fuses) be sure to turn power off before checking them.  If you look hard enough you'll find one or two control fuses (usually on the outside of the machine - Milnors hide there's inside. . .Wascomats hide there's in the back, Uniwash UW's are on the side, Unimac UF's are in the front inside the cover-plate, and UC's, SC's, HC's & AT's are under the top cover). 

It may also be that the on/off switch is no good.

Even though the display lights up, and lights come on, the circuit breaker or line fuse(s) may still be bad.  Most machines run on a 3 phase system (that's 3 separate 120V lines) the motors use all three lines, however, the controls (lights, display, water valves, drain valves, computer) only use one or two of these 120V lines.  So a bad circuit breaker may only be giving two of the 120V lines instead of three.  And likewise, if one of the line fuses (these are the big copper ones) has blown, the controls will work, but the motors won't turn.  So be sure to check these first.  If they check out OK, then it may be in the door interlock switch(es) (this would be the switch(es) located somewhere close to where the door closes) sometimes the switch hangs up or goes bad.

It may also be the start switch (more usually the case with UC's, HC's, AT's & SC'c, Wascomats and Milnors. (this would be a good low-cost spare part to have on hand (see the rec. spare parts list) because it's a whole lot easier to replace it that it is to test it)

If the machine is a newer Uniwash UW series; they install a magnet where the door handle is, this magnet trips the door interlock switch.  Most people do not even know it's there (until they get hit with the service call) this magnet breaks off sometimes so be sure to check it.

This is usually just a broken belt (and most of the time, you can hear the motor turning even though the belt is broken). 

Motor contactors going bad is a common service call. Most of the time though, they give you some warning because they get noisy before they fail. To check open the cover to the motor contactors (usually the top cover), start the machine and watch the contactors to see if they are pulling in, if they are, it still may be a contactor (there are two parts to a contactor: the coil and the contacts.  The coil pulls the contactor in, and the contacts provide power to the motor) you will need to use a voltmeter to check voltage going into the contactor (I've had more that a few wires come loose from a contactor) and the voltage going out.  If you have the voltage in, but not out, it's the contactor.  If you have voltage out, it may be a bad motor, see the motor tech. check page.

IMPORTANT NOTE: If you determine that it is a bad motor and it's an 'Elmo' motor, and you are going to try and have it rewound at a motor rewind shop: be sure that the shop is familiar with that procedure (Elmo motors are European motors... they use a metric winding (not S.A.E.) and they have to be wound by hand, not by a machine) if a shop tries to do otherwise the motor will only last 2-3 months at best.