I'll second what Tom and Chris have said. If you have redirected all your urls to the new site, and the pages on the new site match up in terms of keywords, headings. layout and indexability, then you should have no problems at all.
Here are some things to check:
1. If you do a site:yourdomain.com search in google, does it show all your indexed (old) urls. You must go through each and every one of those urls and click on them to check that they redirect to the correct chosen destination. If these have disappeared from the google index, it might even be too late. I wouldn't be quick to assume that you're in a sandbox ; in my experience that's pretty rare.
2. Have you indavertentyl confused google or done something wrong? Have a look in the webmasters console to see if there are indexing/crawling errors or redirect problems.
3. When you set up the DNN site, did you replicate good SEO on-page factors like using <h1> tags for your headings? Have you gone with a skin that is as good as, or better than, your old html site in terms of clear html and layout, and good use of internal links?
I would be looking at employing the services of a DNN SEO professional at this point in time. Every day that you don't take action to fix the problem could result in the previous rankings being lost forever. I'm not one of those people but Tom Kraak from the post above has handled this sort of situation for sites I know of and rescued it all in time, so you could start by speaking to him.
There's nothing inherent in DNN that is bad for rankings, but it can certainly be optimised through correct use of skins and modules.