Good Evening Emily, 

Thank you for contacting the U of M Extension Service. 

Please click on the following link from the U of M Extension Yard and Garden website:

https://extension.umn.edu/vegetables/growing-spinach-and-swiss-chard This website discusses growing spinach. 

I found the following link: https://extension.psu.edu/growing-spinach-a-cool-season-vegetable#:~:text=One%20key%20to%20successfully%20growing,F%20to%2068%C2%B0F. 

 This website states: 

"One key to successfully growing spinach is to plant the seeds when soil temperatures are cool. Spinach seeds germinate best in a soil temperature of 45°F to 68°F. Recommendations are to plant spinach about four to eight weeks before your average last frost date in the spring and six to eight weeks before the average first frost date in the fall. Two crops per year can be achieved! Many home gardeners have become discouraged growing spinach because they start too late in the spring. Understanding the relationship of spinach to temperature is critical to achieving success."

If your seeds had started to germinate in the warmer weather and then froze, they may no longer be viable.  Also if they were damp/wet when the temperature was 12 degrees, this might be a problem. 

You might either wait a week or two to see if the seeds germinate, or you could plant some new seeds in your greenhouse, after tonight's snowstorm, when the termperature moderates.  I'm sorry I can't give you a specific answer. Cool season crops do grow in cool weather, but our March and early April weather can be a roller coaster of temperatures.  I am curious what the soil temperature is in your greenhouse, with the cover on, at night.  

Good Luck!