If the SIM was working before, it sounds as though it has been activated, but could have developed a fault. The next step would be to try another one (someone else's) and see if that works, but I would expect it to if your SIM doesn't work in a different phone. If you can, drop into a Vodafone shop as they'll be able to test everything and give you a new SIM if that's what's required.
Note that you'll need identity for a SIM swap and that, if the account is in your mother's name, she'll need to be with you (if the account is in your name, but your mother is just paying, you should be OK, though).
