Hi Kirsty,
You mentioned credit, so I'm guessing it was a pay as you go setup that you were previously using successfully?
If I'm right in thinking that, and you haven't used the device and sim for a long time then it's possible that the sim is no longer active. You could find this out by either calling Vodafone customer services or putting the sim in a mobile phone (not network locked if from another carrier) and seeing if you get signal. If you do key *#100# and press the green/send/call key... you should get an 07XXXXXXXXX number appear on the phones screen. If none/only one of the above happens then it's likely to be a sim related issue and either getting a new sim or having the one you have reactivated would be the way to go. Both mean calling Vodafone customer services or maybe going to a Vodafone shop.
It's possible that no network is being found due to a lack of Vodafone signal or low signal strength. The USB modems aren't quite as good as picking up signals as phones due to their small ariels (like many things in life, bigger is better when it comes to radio and antennas). If you get a good (like 3+ bars) Vodafone signal on a phone, then it's unlikely to be signal strength or coverage causing your problems and the first paragraph of this post is more likely ot apply.
If you do get a new sim it's possible that you'll have to reconfigure the Vodafone connection manager software to use the APN associated with your new sim. How you do this depends on the version of Vodafone software you have. The APNs are dicatetd by the type of account associated with your new sim:
internet (if you are a contract/pay monthly customer)
smart (if you're a pay as you go customer on a £5 for 250MB lasting up to 30days plan)
pp.internet (if you're a pay as you go customer on a £15 for 1GB, lasting up to 90 days plan)
ppbundle.internet (if you're a pay as you go customer on a £15 for 2GB lasting up to 30 days plan)
in all cases the username and password are both: web
Hope that helps.
