I'm a member of Bike Share Toronto. This is one of various bicycle-sharing systems in various countries which use PBSC Urban Solutions as their technology provider.
I went to the Bike Share Toronto website, and logged into my account. I was able to log in successfully. However, above my name and address, I saw a brightly-colored message banner:
"The subscriber is valid but the account is disabled."
Why did I see this message?
It turns out that I saw the message because my old credit card had expired, and I had not entered the replacement credit card's details into the website.
When your credit card expires, your account is automatically deactivated. You cannot unlock any bikes anymore. If you have the PBSC app on your phone, you will automatically be logged out from the app as well.
PBSC does not send you any email or push notification to inform you of the deactivation. It just happens silently.
You'll normally get a new credit card in the mail about a month before the old one expires. (Source.)
Once you receive your new card, don't forget to log into your Bike Share account and to enter the new card's details. Otherwise, you may get an unwanted surprise one morning, just when you were about to begin your daily commute.