it could be many reasons...
1) you've screwed up the URL structure of the ranking pages and / or didn't do the 301s correctly...
2) the content that ranked is completely changed and now google doesn't consider the new content as being relevant / quality anymore...
3) sometimes, changing the theme of a website can cause it to lose rankings, but the good news is that - also sometimes - the rankings come back on their own, so you probably need to wait
4) your site is fresh (less than 1 month old) and - like most fresh sites - there's a 1 month "honeymoon" with google and after this honeymoon is over your site goes in a limbo of sorts, and the ending of the honeymoon simply coincided with your re-designing of the site, which makes you (inaccurately) assume that it's the re-designing that's caused the issue when, in reality, it would be the 1 month free ride that's ended
5) you've been affected by one of google's gazillion updates, in which case you're probably screwed for good, and you either have to resort to black hat stuff or - most likely - give up on this site and start afresh
6) you've been outranked, in which case you need to build links until you go back on top
7) your site is not / had not been mobile friendly and google doesn't like it, which is why they've kicked you out of the SERPs, in which case you need to make your site mobile friendly, resubmit the sitemap to your GSC (not needed, but it can help with faster re-indexation / re-ranking), and hope for the best
8) and speaking of re-indexation, it's also possible that your site had been de-indexed for whatever reason (malware / phishing, manual penalty, DMCA takedown, illegal niche, etc), in which case you're screwed again and you can abandon the site
These are just some of the reasons that I can think of based on the little to no information that we have about your site, so look into each one and fix accordingly. Or run a full site audit and competitor analysis (or hire someone to do this for you if you don't know how to do it yourself) to identify possible causes for your site's downfall which could very well be caused by the redesigning itself (many times this can be the case)...
