It used to be that when an expired member logged in, the page would display "your membership has expired, please renew" and there would be a link.
That should still be the case unless "Membership expired redirect URL" was configured within CBSubs > Settings > Display > Expiration. The plan also needs to still be accessible to them (see Access tab of your plan) and allow for renewals (see Pricing tab of your plan). You can also provide what you have the parameters within "Pricing and validity" on the Pricing tab configured to to see if there could be a configuration problem blocking the renewals.
Additionally you can configuration CBSubs to allow free registrations without a subscription within CBSubs > Settings > Global so it won't block them anymore.