Firstly, have you got an active goodybag, or just airtime credit? As there is a difference, and often people get confused between the two (please skip past this if this is obvious to you!):
Airtime credit is just that - money in your account to use as pay as you go (5p/MB, 15p/min, 5p/txt) or for items outside a goodybag if you have one active - calls abroad and premium rate calls for example. Airtime credit does not expire, and stays on your account until used.
You can then choose to buy a goodybag if you wish - either from your airtime credit, or using new money from a card or Paypal. This is a bundle of data, minutes and texts that last one month - and then disappear whether you used them or not. You can then choose to either default back to pay as you go, or buy another goodybag.
If you do have a goodybag, try dialling
*100*7# to check it is active. Maybe also try restarting your phone as that is often needed to get a goodybag to kick in.