If you happen to have a DC voltmeter handy (ha), then you can test the supply to make sure it's still putting out 12 volts. If the power light is just plain off, then it's very likely the power supply has blown. Cheap Chinese wall-warts tend to do that a lot - and not just AT&T's.
The good news is that you probably can buy a replacement at a local electronics store for under $20. Just make sure you get one that's 12 volts DC, center positive (and with a connector that fits), and puts out at least 1000 mA (or at least 1A).
Another solution is a 802.3af PoE injector/splitter pair. This has the nice side effect of letting you run just one cable to your Microcell for both Ethernet and power.