You can reheat your crispy hash browns using a preheated oven at 350 F.
I reheat my crispy hash browns by preheating my oven to 350 F.
Then while the oven is heating up I wrap the crispy hash browns in some tin foil and then place them in a baking dish.
Then I let the hash browns heat up for around 4 to 5 minutes which is usually enough to get them warmed up without burning them.
If they are not warm enough then you might want to heat them for 10 to 15 minutes depending on how cold they were to begin with.
You can also heat the hash browns up in a skillet or in a the microwave and if you do put them in the microwave you should add a class of water to keep moisture in the microwave to prevent the hash browns from drying out while heating them up.
In the microwave 1 or 2 minutes should be enough to heat the hash browns up.