Hi Jane and welcome to the forum. Mine does that and i'm not sure why but I have always managed to immediately fix that by disconnecting the WiFi/modem router, wait about a minute and then reconnect it. Try it, it may help.
Regards, Andrew
The first thing that I would try is rebooting your router, then forgetting the network in Settings on your iPad and re-sign in. If that doesn't work you could check that your routers firmware is up to date.
There is lots of good advice and info on here plus plenty of members all too willing to help with any problems.
Remember, search is your friend. If you can't find the answer you need and you post a question then give as much info as you can to help members help you i.e. iPad 1 or 2, etc. and iOS version and name of app if the problem is with a specific app........and please don't forget to read the forum rules http://www.ipadforums.net/forum-rules-help-info/2119-forum-rules-everybody-please-read.html
If you have other Apple products or are interested in other Apple kit then why not join our sister forum too at http://www.appleforums.net/ and for iPhone users, come and join in the discussion over at http://www.iphoneforums.net