I've had something similar within the past month. Posts, pages, images and plugins I uploaded that day would be gone the next. And at times, I couldn't log into wordpress. Or the site just wouldn't come up. When I checked my ftp program, anything I uploaded was there but I couldn't access it online.
This was happening to about 10 sites - all with the same hosting account, but on different IP's. Also, my sites would not show up for spiders - page rank of 4 dropped to 0 on a 7 year old WH site. But then the sites would appear after a day or two and then it would disappear again.
I changed passwords, looked for signs of being hacked, but couldn't find any. I checked that I wasn't over bandwidth, that the server wasn't having issues, emailed and spoke with support many, many times.
Finally I decided it was the hosting account and now am almost done moving all of my domains to a new host.
Good Luck with your problem - hope this helps