Have you checked logs to determine if it was some type of DDOS attack?

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>We had some problems which were difficult to resolve. Even with what I did tonight, it is not clear what was causing/is causing this. So far, with what I did in the last hour, everything seems to be ok. But, I'm not sure exactly if this will run ok. We are watching IIS which looked weird and will probably upgrade it if it continues to act weird. There might be corruption that happened over the weekend. What we observed was that is was going slow for a while and then back to normal. On some occasions, a no access was applicable between 10 seconds and 4 minutes.
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>As for Chatter, it's probably because the login didn't work well and the next one simply overwrote the previous session with new message. So, you might use Time Warp, the top toolbar reports or the Calendar to retrieve them back.