We can calculate that. Since the air is gone convection cooling [1] isn't relevant anymore. I'm going to ignore conduction cooling [2], assuming the greenhouse is isolated from the lunar surface. This leaves us with radiative cooling [3]. Conveniently radiative cooling can be calculated using these online calculators [4] [5].
The damage won't be equal across all the plants, so I'll run the numbers for a few examples. The greenhouse will be run at 301 K, a temperature that came up when I googled optimal greenhouse temperature [8]. I will assume the plant to be dead as soon as 273 K are reached, as water will freeze here.
Cabbage and Head Lettuce
Assumptions:
$radius = 15 cm$ average cabbge radius
$density = 362 kg/m^3$ bulk density of cabbage
$molar mass = 18,02 g$ molar mass of water (there's a lot of water, plant mass and air; thus should add up to water)
Cooling Time:
$ca. 900 s = 15 min$
Apple, Tomato and Fruits
Even when the plants are dead seeds could be salvaged to grow the next generation.
Assumptions:
$radius = 5,3 cm$ average apple radius
$density = 740 kg/m^3$ density of apples
$molar mass = 18,02 g$ molar mass of water (there's a lot of water, plant mass and air; thus should add up to water)
Cooling Time:
$ca. 660 s = 11 min$
Seedlings and buds
I'm assuming to deal with a roughly spherical seedling meaning a very young one.
Assumptions:
$radius = 0.1 cm$ average seedling radius
$density = 600 kg/m^3$ average density of leaves
$molar mass = 25 g$ molar mass of water plus ca. 7 g
Cooling Time:
$ca. 7,3 s$
leaves
Assumptions:
$thickness = 0.1 mm$ average leaf thickness
$density = 600 kg/m^3$ average density of leaves
$specific heat = 1,76 J/g*K$ value for wood, but I could find nothing better (I'm not sure if the higher water content of the leaf will increase cooling time or if the difference in material will worsen it.)
Cooling Time:
$ca. 7 s$
Other thermal factors
Not all plants will die at the same pace and my calculations just show when frost damage will start. Leaves, seedlings and buds will reach 257 K (-20 C) after around 15 s. My guess is that thats the point of no return concerning damage. Be aware that roots and trunks could survive way longer, the former because they are buried (assuming the greenhouse uses dirt instead of nutriant rich water, which would make things way worse for the roots (see evaporative cooling [6])) and the latter because they are thicker. Yet some plants could be lucky enought to be saved at 2 to 5 times the limit I guess. Additionally I assumed that the plants can radiate heast away freely. In reality the plants and the structure will radiate heat at each other and since the moon base will be insulated, truly losing the heat could take some time. This might increase the timescale by a few orders of magnitude. Keeping the heat lamps, lights and radiators on will counteract the heat loss as well, potentially making thermal damage improbable/impossible.
So unless the plants end up outside the greenhouse or the roof is ripped of freezing seems to be out of the equation.
Conclusion
Be aware that temperature won't be the only source of damage. Decompression will take a toll, depending on its speed and exposure to vaccuum itself will hurt due to evaporation. This experiment [7] exposed plants (radish, lettuce and wheat) to 0.015 atm pressure for 30 min. They wilted a bit due to dehydration, but grew in fine afterwards.
So vaccuum doesn't seem to borther the plants much and your colonists are in no hurry to restore the greenhouse.
This is disappointingly anticlimactic...
[1] https://en.m.wikipedia.org/wiki/Convection
[2] https://en.m.wikipedia.org/wiki/Thermal_conduction
[3] https://en.m.wikipedia.org/wiki/Radiative_cooling
[4] http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/cootime.html
[5] http://mc-computing.com/science_facts/RadiationBalance/CoolingCalc.html
[6] https://en.m.wikipedia.org/wiki/Evaporative_cooler
[7] https://www.google.com/amp/s/www.newscientist.com/article/mg20927953-500-vacuum-of-space-no-match-for-the-mighty-radish/amp/
[8] http://www.just4growers.com/stream/temperature-humidity-and-c02/understanding-the-optimum-temperature-for-plants.aspx