Dehydration makes the natural balance of water movement in and out of eye worse, that brings the lens a little nearer to the retina making your vision longer sighted than normal.
There is then also age related muscle denegeration also making your eyes longer sighted.
There is then illness related such as diabetes, macular degeneration etc.
If temporary it is probably therefore dehydration. Avoid alcohol, drink water, etc.
The migraine blindness is easy to head of its worse effects if you can get quickly to a dark quiet room and close eyes, chill down and the frazzle in your neurons has time to go away. Migraines in my case look like this:
They usually appear at edge of vision and gives me time to shut down my plans and get to a dark quiet place, but if I do not they gravitate to my central vision then I'm in effect blind and rely on telling someone and get their help, but being incapacitated has not happened, I think for > 30 years as I know the signs and what to do now.