HELP! My Crock-Pot lid was Slightly askew while cooking my roast!
We don't know from your question how long it was on low with the lid askew (we don't know your time zone) so it's hard to say for sure.

But usually a small crack on the lid isn't going to really really drastically reduce the temperature. You're in a better position to tell than we are:


If it was still bubbling and steamy inside, even with the lid offset, then you're probably fine. Maybe it took a little longer to come up to temperature and will be done slightly less quickly, but it's unlikely to be a disaster.
If it wasn't, then you have to worry about food safety too: if it was like that for more than 4 hours, it may have been in the danger zone (under 140F) for that long, and thus be unsafe.