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Most all purpose fertilizers will be ok. These should be fertilized twice a year. Here is an article for more information on their care: https://www.gardeningknowhow.com/ornamental/trees/windmill-palm/growing-windmill-palms.htm

The only thing you can really do now, is let it recover. The problem is that it is hard to know if it stayed cold enough to effect the roots. These plants can only survive down to 13 degrees, and not for very long. so during these times, it is best to cover it with burlap until the temperatures let up some.

Just care for it as you would normally. and it has a chance of putting out new growth when it warms up some.

Here is an article for the care of these: https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-care.htm

The old, and dead fronds can be cut off, but it is not totally necessary. This article will give you more information: https://www.gardeningknowhow.com/ornamental/trees/windmill-palm/growing-waggie-palms.htm

As long as the soil is, generally, fertile in your area then there will be no need to fertilize. If you noticelack of growth for long periods, or yellowing and browning of leaves then you can add a palm fertilizer. Otherwise the care is minimal. This article will help: https://www.gardeningknowhow.com/ornamental/trees/windmill-palm/growing-windmill-palms.htm

Your watering habits, as of late, will be the culprit.

This has gone fungal. There will be a few things that you can try here, though. The first will be a LARGE HANDFUL of DOLOMITIC LIME and a TABLESPOON of WETTABLE SULFUR per plant can help the infection.

The next options will be fungicide sprays. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

This article will help you to care for these palms: https://www.gardeningknowhow.com/ornamental/trees/windmill-palm/growing-windmill-palms.htm

For 2 to 3 weeks after you've planted your palm, keep it well watered. Wrapping a soaker hose around the root area will keep it moist while saving water. Add enough water to ensure moisture is penetrating a foot and a half of the soil. After the plant becomes established, you can cut back soak-watering your palm to about 15 minutes twice a month. Cut back on watering during the winter months.

Palms have nutritional needs that other trees may not. They depend on ample supplies of magnesium, iron and manganese to keep fronds from yellowing or curling.

https://www.gardeningknowhow.com/ornamental/trees/windmill-palm/growing-windmill-palms.htm