It may happen that your upload stops if your machine has turned off, or that putting it on standby turns off your hard drive and/or your Internet connection (these options can be configured on your computer).
In order to resume your upload, all you have to do is refresh your page, take your file and drag and drop it to the place where you uploaded it in the first time. Then you will have to click on « Add ».
NB: if you uploaded your file from an external hard drive and your upload has stopped, the recovery will not be possible. The upload will then resume from 0%. If your upload stops at 99%, it means that the MD5 signature is not done (network micro-cut, resumption, disconnections between the hard drive and the computer). Dragging the file back will be enough for it to be finalized.