Search Console groups URLs together and assesses the entire group's CWV performance. So for all URLs to go from "good" to "needs improvement" all at once, that suggests two things:
1. All or most of your URLs were placed into a single group
2. The aggregate CWV experience for one or more metrics were very close to the "good" threshold, so a small regression in performance or a change in relative page popularity tipped the group-level aggregation over the edge to "needs improvement"
The good news is that it should be just as easy to tip the URLs back to "good" as it was to "needs improvement" assuming that the aggregation is now similarly just beyond the threshold.
The next step I'd recommend is to look at your most popular pages and assess the biggest opportunities for improvement. For example, if you have a very popular page with "needs improvement" CLS, optimizing that page may be enough to get the entire group back in the green. That said, it's a precarious position to be in, so continue to focus on improving your site's performance to avoid being so close to the threshold.
Disclaimer: I haven't seen any data for this site, I'm just guessing what's going on based on the description of the problem.