Have you squeezed it multiple times until it felt back to normal/your liking? If not, describe the feeling.
It could be air/moisture in the system. It could be attributable to the weather. Brake fluid will be have slightly differently at 90 ℉ comapred to 50 ℉. How old is the brake fluid? Do you see any leaks?
I had recently changed the entire fluid, bled out the system before putting the bike away. As suggested, I have pumped the lever a few times to get some pressure into it. However, I have one those after market levers which are adjustable and it now requires the highest position the get any reasonable pressure feel.