Our site came down Tuesday afternoon (PDT) was it you or us?

I was notified about 4pm (PDT) on Tues July 31, that our site was down. If this was caused by us, is there a log I can check for activity around that time? The server came back up about 8pm that evening. I wasn't on or making any changes to the site when it occurred. As a matter of fact I was at Frys buying a new router when it came up. I hadn't made any changes to the site in over 36 hours but had visited the site many times since making my last changes. I use dreamweaver to build our site. I have been the webmaster of this site since July 1, 2012 and have recently discovered that there is someone else using our site...I am not a happy camper about this. If I could isolate what was going on just before the dump, it would help. I am not getting any error messages but I don't want this to happen again. I cannot replicate the problem. My computer was in shop being disagnosed at the time and I had no internet access. I was notified off the site status by phone from one of our members.