Like many things, BBM is a proprietary-BB service, and requires subscription to an adequate data plan to activate it, regardless of the pathway used (carrier data network versus WiFi) to access it. Since you let your data plan lapse, the services associated with it also lapsed...including your access to BBM.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.