Well assuming you have a backup just revert back to the old site, and it should return.
Mass 301 is not an ideal solution and should be avoided, too many webmasters have abused 301 redirects and Google now doesn't pass all juice and if done in the masses can have a negative effect.
Futhermore, there's another thing which should be avoided, mass changing of titles, h1 tags, and metas and the actual page content of the pages. A redesign should be that of just the look, the content should be done very slowly.
Google has Patents that 'Monitor' Title, Headers, and Other changes on the page to try and manipulate the results, changing these can be very damaging and should be avoided if your site is ranking fine. Don't fix something which isn't broken.
A redesign is one template to another, it sounds like you gone from one site to another, which isn't a redesign but a whole new site.
The correct way of going from a old site to a new site is:
- Change the look, keep URLS, H1's, Titles, Page Content, Metas, Alt Tags Everything the same.
- On the pages your not ranking on, simply delete these and make a 301 redirect to the new page with new content, new headers, new urls etc etc. This will retain any links. If you do not have any links then simply just remove it all together, no point in using a 301 unless you have juice going to it.
- If you have pages that are ranking, but you want to make these pages stronger with new content, then do it slowly and try to keep the page title, headers and meta the same, again... if its not broken don't try to fix.
- If you have loads more pages of content to add, then add it slowly. Adding content over a set period actually improves your Google Crawl rates as well as boosting your rankings for being an active site.
Bottom line is a lot of change all at once is a bad thing, moving to a new site should be treated as a very slow thing.