What is the make and model of the phone you are using and was it purchased from Fido?
Here are a few things you can try.
Try restarting the phone
Ensure data is enabled even if you don't have data that option needs to be enabled because if your messages are converted to an MMS the data needs to be enabled.
Check and make sure the APN setting is correct on the phone you can verify those settings here.
If nothing works try to contact customer support and ask them to do a reset to switch and see if that helps. It's better to call from another phone if you can because once they do the reset you will be disconnected, once they do it restart the phone and see if your text works.