Half a year ago I made my site unavailable, and I reactivated it again just now. It seems I have fallen off google completely. Will I be penalized for this, or will google give me a "clean slate" as it were?
You will only be penalised if Google found issues with your domain that fell outside of their guidelines that were serious enough to have raised a flag.
Don't expect your rankings to reinstate though, you'll have lost most, if not all authority that your domain had. You'll have to wait for any remaining backlinks to your website to be crawled and for Google's algorithm to score the content of your site and the backlinks that pass any value to your website.
It may help:
You should give Google some time for recrawling your site. You will lost most of your positions in serp.