google is not sure you own your domain anymore and that is was possibly bought by a spammer who looks for expired domains. you want to demonstrate to google that ownership has not changed. one way to do this is to not change the page templates, site structure or hosting company.
i have had some good & some bad experiences with trying to get expired domains ranking again in google. i would suggest you get some new backlinks while you wait to see if google gives you credit for the old links.