At the beginning of the summer I had around 1,172 reputation points. I didn't log on for a few months. Then, I saw that the site looked a little different and I looked at my profile and saw that I only had around 650 points. I was worried to log on because of the points and because the login experience seemed different - like I was creating a new account. Yesterday, I logged on anyway and again saw that I had around 650 points, though I think all my questions were there. Then, today I was going to come to meta and ask why I lost so many points, and when I logged on I only had 1 point and no questions! Does anyone know what happened? Can I get my account and questions back?
Perhaps I should answer in more detail as there were several questions asked:
The reason the site looks slightly different is that the software was upgraded and MathOverlow is now part of the Stack Exchange Network (together with math.SE, Stack Overlow and various other sites.) See here for the thread celebrating it Why is the update so awesome? and here for a thread on the old meta (now tea) in preparations http://mathoverflow.tqft.net/discussion/1416/mathoverflow-20/
The change of reputation is as Gil Kalai said mainly due to the fact that since the upgrade an upvote to a question gives only +5 instead of +10 and this was applied retroactively. There are some minor additional effects that play a role see Has there been a recalculation of reputation? for some details. This change happened on all sites in the Stack Exchange Network long ago, and now that MO joined also on MO. The general/official reason (not MO specific) for this is to be found here http://blog.stackoverflow.com/2010/03/important-reputation-rule-changes/ .
The reason you now created a new account when logging in while using the same Google account is due to to some technical change of the way Google Open ID is handled, see How can I log in into my old mathoverflow account? for how to get your account back and why the extra step in registering? for more details and a side-effect of this that might or might not affect you.
So, in brief, it is MO with a software upgrade, and you can get back your account following the instructions above. The slight reduction in points, while unfortunate, is also expected and nothing specific to your account.