If your account was recently disabled at the end of your trial period and you have since re-enabled it by choosing a plan (free or paid) you'll need to ensure that you also re-enable your Slack integration (if you were using Slack previously).

You can do that by visiting the Integrations page, finding Slack then switching the grey toggle at the top right of the card so that it's green.

This will re-enable your Slack notifications.