Whatever it's worth, I had the same issue. At first, I thought it was the keystroke binding on my mouse - it wasn't. I was stuck in sneak mode no matter how many times the sneak mouse button was pressed - nothing. I did find an answer. For some reason the game thought I was trying to do another action. I simply went to an object that would cause me to "stand". e.g. alchemy table, mining ore, etc. Issue was resolved.