Originally Posted by zacster
I would put the battery on a voltmeter and check the voltage after letting it charge. It should read 7.4v.
It should read between 6.0 (as low as they should ever go) and 8.4 volts (fully charged.)
But more to the point, the plugs these use are not conducive to the use of a voltmeter unless you get a male plug to plug into it and then measure the voltage on that. If you stick the voltmeter probes into the plug, you're quite likely to short it out, even if you know exactly what you're doing and have very steady hands. And shorting it out, even just for a second, can easily damage the protective circuits.
If you can't get a matching plug, you're better off cutting off the outer layer of insulation on the wires and putting your sharp probes into the inner wire's insulation and measuring the voltage that way.
In any event, cyccommute's comment is probably right on -- it's probably gone below the voltage that the protective circuit will allow you to charge it. If so, the battery could probably be revived by cracking it open and bypassing the circuit and charging the individual cells back up past that level, but there's a risk to that too -- the circuit prevents charging an overly discharged cell because the cell is likely damaged by that. If it's only overly discharged by a little, it's probably OK, but the circuit is playing it safe.
That said, you can buy replacement batteries fairly cheaply. The $16 Magicshine clone on Amazon includes a battery that's probably pretty much identical to what you had, and it comes with a charger and light head to boot at that price. (And it's also possible that your charger is what's bad.) The only possible gotcha is that the plugs may not quite match, and you might have to cut some wires and move plugs around or replace them. And there are other replacement batteries out there as well.