@lorastein
If you didn't initiate a number change recently, then it sounds like your number may have been re-assigned to another Telus/Koodo/Public Mobile (PM) user by mistake, which has happened in the past. This is a screw up on PM's side of things, and there's nothing you could have done to prevent this situation from happening in the first place.
For whatever reason, the Telus/Koodo/PM computer system mistakenly thought your cell phone number was not assigned to any account, even when it was assigned to your account. Unassigned numbers are available to other Telus/Koodo/PM users, so your number was most likely re-assigned to (or chosen by) another customer.
You will need to contact customer support and explain your situation to get your original phone number back. Since you did not willingly change/relinquish your phone number, PM is obligated to return your cell phone number to you and assign a different number to the other customer. However, it may take PM a few business days to investigate and return your phone number, so you will need to be patient in the meantime.
Please follow the instructions provided by Yummy to contact customer support. If you have never contacted customer support in the past, here's an abbreviated version of what to do. I would strongly recommend asking a senior CS_Agent to handle your issue, since getting your phone number back is a time-sensitive matter.
