You could try adding a bit of water, which would cause it to heat up again, and potentially dissolve the solidified layer. However, the problem is that all those hours of contact with air has converted some of the NaOH to sodium carbonate (soda ash). This results in a much weaker lye solution than what you originally created. There are ways to test your solution to determine the current (degraded) strength, but that's above my pay grade. It it were me, I would pour the batch down any slow-moving drains, and start over.
For future, I actually cover my MB lye solution almost immediately after mixing. That way, I don't lose any water to condensation, and I don't lose any NaOH due to conversion into soda ash. My container of choice is a repurposed laundry jug, so it's easy to loosely cap it while the solution is cooling, rather than completely tightening it. That way, there is some room for pressure to escape if needed.
