is it possible to activate a old sim card?
The sim card i have has been activated and now deactivated for quite a while.
I tried activating the SIM card on the web site, but it said the card was invalid
Provided the SIM isn't so old that the network can't support it, a SIM can be re-activated. I'd call CS directly and have them do it. That way, you'll be assured it was done properly and how much time needs to elapse before it's re-loaded into the system. Of course, if Fido have some company policy not to re-activate, I can't suggest another approach. Theoretically, it's not difficult to do.