When you say "none of my documents are on my desktop" do you mean that the desktop is completely blank? No Icons of any kind? So all you can see is your dock and the menu bar?
If that is the case Finder may not have loaded or might have just crashed and got stuck at login.
try this: Hold Command + Option and tap the esc key. That should bring up a dialog box with the option to kill any running tasks. Finder may be in there with"(Not responding)" after it. If so click on it and click on the Relaunch button.
If that box is empty then you have a more serious problem or if relaunching does nothing. Sometimes when you upgrade the O/S you have incompatible software installed. That could be the case here. Power off the Mac (hold the power button till it turns off) and then turn it on while holding the Shift key. This is called booting in Safe Mode.
Once you get the grey Apple logo release the Shift key and let the Mac boot. Safe mode turns off most, if not all, 3rd party additions to your system. It also does extensive disk checking so startup in Safe Mode can take substantially longer, depending on what is going on with the Mac. I've seen it take all night.
If you can boot up and log in under safe mode then there is something installed that is incompatible with Yosemite that will need to be uninstalled.
If safe mode does not work there could be a corrupt installation of Yosemite a disk error or something else that might take an experienced Mac technician to properly diagnose.