Did you start a big upload and walk away to let it complete while you tackled other things? Make sure you disable your Mac's sleep mode to ensure your upload doesn't stop in the middle.
To do this, select System Preferences from your Apple menu.
Click on the "lightbulb" icon for the Energy Saver option.
Select Battery, and move the "Turn display off after" all the way to the right to "Never."
Exit your System Preferences and start your upload without worrying about it stopping mid-transfer. Once you're done, don't forget to go back to your System Preferences to change the battery setting back to your preferred setting.