Okay, I had this happen once a couple of weeks ago. Here is what I did to get it started again. I don't know why it worked, but it did.
I went into the Photos app, and opened Settings. I scrolled down to "Auto upload" and clicked where it said, "One Drive - turned on". I changed all the settings to Don't Upload. I powered off and back on again. Then I re-entered the settings and chose Best quality for Photos and for Videos. I powered off and on again.
Next I went into "upload status". I tapped on a photo waiting to upload by Wi-Fi. It gave me the options to "connect" or "not now". I chose "connect" and it started to work.
Now, while this worked for me, I make no guarantees it will work for others. I'm using WP 8.1 GDR2 on Lumia 635, so there may be some OS differences that made it work for me, but not for you. Still, worth a shot, and way better than a hard reset.