I'm sure other people would have more insight, but if your wifi is showing up in the notification bar (the area at the top of the LCD screen... active wifi looks like a dot with three horizontal arcs above it), it should be on.

Go into Settings>Network and Controls. Is the green check next to "Wi-Fi" on? What does it say directly under that (eg, connected to XXXXnet)?
Touch "Wi-Fi Settings". Under the gray bar labeled "Wi-Fi networks", what do you see? This area should show any available networks. If the appropriate one is listed, your wifi is working, I would think.

If nothing else, providing this information might help others on the forum figure out your problem.
Good luck!