Weird iMac issue

[shakalaka]

So for some reason I am having a weird issue with my iMac at the office. Yesterday I changed the desktop picture (wallpaper) to what I wanted and turned the computer off. I turned it on today and it was gone to one of the stock wallpapers the computer has. I tried changing it again and now the whole screen is blue. I am not sure what's going on?

When I go into system preferences, it shows the wallpaper I set yesterday there in the display box but the actual wallpaper is all blue. Can anyone help please?:dunno:

[revelations]

Way too little information:

what OSx version?
what programs running in the background?
rebooted a few times?
recent phenomenon? (appeared after new program install)

[shakalaka]

For some reason it started working on it's own. I don't know if it was possessed or what happened but I set the same picture as desktop again and it worked this time. Weird.:dunno:

Just to answer your questions. I had nothing new installed what so ever. I had tried rebooting it once and the same issue happened. Nothing running in the background other than Safari and I have Yosemite.