Mower won't startEdit
More common solutions:
Check battery is chargedCheck power switch and safety key are both on
Mower loses power too early (< 30 min or so)Edit
Is your battery good?
Lead acid batteries require several things for long life and good service. Keep them always fully charged-plugged in. Try not to use the mower until the battery is used up (or at least don't do this often!) I've read that batteries last longest when they are not used past 50% of their capacity, and last even longer if only used to 25% of capacity. Of course, you acquired your mower to use it, not to look at it charging!
Is your mower motor in good condition?
Most mower motors have carbon brushes that transfer electricity from the battery/control unit. The brushes should be in good condition and able to freely move in an out (with springs applying good pressure).
The motor's axle (and blade etc) should freely turn. (Observe safety steps including disconnecting battery terminals, removing safety key before you even think of putting your hand near a mower's blade) Even if the motor does freely turn, if your mower is old it might be a good idea to oil it. The motor's upper and lower bearings should be lubricated (just 3-4 drops of high quality oil (not 3n1 type stuff; use Mobil 1, synthetic, or oil made for lubricating small motors) at the top and bottom. On some B&D motors there is a little felt reservoir. You can put your 3 or 4 drops there. You can turn the mower upside down (observe safety steps including disconnecting battery terminals, removing safety key, inserting 2x4 stud into mower blade to insure it can't turn while you use a wrench to remove the blade etc), remove the blade and blade positioner to get at the place where the motor's axle disappears behind a bolt. Inside there is a bottom bearing that can also take 3-4 drops of oil. Leave mower upside down for a while. Don't get oil on other parts of the motor, such as the brushes area, the axle itself or the copper parts of the armature.
Could your electronic control module be shutting down due to a 'false overload'
The only part of the mower that isn't simple to diagnose is the electronic control board. This unit (which has the charging port and charge indicators on it controls the charging process for the battery, the overload protection (if the amperage draw of the motor becomes to high and the 'instant stop' of the blade when you release the power switch. It also is the receptacle for the safety switch and the relay for the small switches to control a large motor.
During normal operation, the B&D CMM 6xx series mower seems to draw about 20 amps when spinning the motor and blade. Under wet or tall grass conditions this increases. The control unit shuts the power off if it senses either too high a current draw or, possibly, too low a battery. If you are able to measure the current draw (you can use an inductive clamp-type meter) and see the draw is much higher, suspect a motor that needs lubrication (see above), dirt or gunk in the motor or brushes, or a faulty control unit part.
If anyone has the circuit diagram/schematic for the control board, please post it or a link to it here - as we don't have one yet!