I started my pot roast on low at 6:30a... I just realized the lid was askew, just a bit. I have cranked it up to high...with it be done by 6:30p It is not a big roast...maybe 2-3 lbs.
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.