Power off the Express and your computer
Wait a few minutes
Power up the computer for a minute or two
Power up the Express
If that does not work, then we have to start asking embarrasing questions like......
What is the model number of the Express? (On the side or bottom....starts with an "A" followed by four numbers)
What operating system is your Mac or PC using?
How was the Express configured to operate on the network.....what does it do when it is working?
Any other Apple AirPort routers on the network? What model numbers?
Please assume that we know nothing about your network (we don't) so we will only know as much about your problem as you can tell us.