Several possibilities, but I sincerely doubt you did anything to cause your Express to break. They are pretty abuse-proof, but it is possible an internal parameter has become corrupted through no fault of your own. The only way to fix that is with a "hard reset".
While you do that, it's a good idea to reset your cable or DSL modem as well. Make it a rule to reset your modem whenever you connect something different to it. Failure to do that could conceivably explain the problem you are experiencing.
The following is an excerpt from
- Press and hold the reset button with a pen or pencil until you see the status light (LED) start to flash amber rapidly, which should occur after about five seconds.
- Release the button, and the AirPort Base Station or Time Capsule will hard reset.
... but read the entire document to ensure you do it correctly. An Express that has been "hard reset" will create an open network with the name "Apple Network nnnnnn" where nnnnnn = the last few digits of its MAC address.
If you do not see the network with that name, the "hard reset" did not take effect, which will happen if you do not hold its little reset button long enough. In that case do it again.
Following the "hard reset", reconfigure your Express.
It is also possible that your Express has simply experienced infantile failure. It happens.