It’s a bit difficult to say. A picture might help. Once the pool is filled the sides will straighten out and then there is very little pressure on the corners. They will normally slide in and out a bit as the pool is used (although they are long enough that you can’t remove them completely when the pool is full. So if it’s in far enough, then once the pool is filled then it might get to a point where it is loose enough that you can just slide it back in.
However, on the other hand, if it’s not in far enough, then it’s possible that you could get the pool filled and still not be able to get the corner into place and wind up having to drain the entire pool to fix it.
Additionally, the corner pieces are under a good bit of stress as the pool fills because the walls try to cave inward and the corner pieces are what keeps them from completely collapsing. This stress is going to get worse and worse as more water is added to the pool, until the point where it is nearly full. Worst case, if the corner piece is just barely hanging on then it might slip out and cause the pool to collapse and get damaged.
So if you think the corner piece is mostly in place then you might be able to get away with leaving it and continuing to fill, however, there is some risk that once the pool is full you may not be able to fix it and will have to empty the whole pool and start over. And if it’s barely hanging on then there is also some risk that it could slip out completely as the pool continues filling and damage the pool.
As much as it sucks to do so, I’d probably err on the side of caution and drain the pool enough that you can fix the corner and then start filling again. I don’t think you’ll need to completely drain it, but you‘ll probably have to remove a significant amount of the water.