How long has it been since you harvested the plant?

It has gone to sleep for the winter. Leave it be and when the weather warms up again, come spring, it will bounce back with renewed vigour. Even in my subtropical climate, lemon balm and many other plants go into dormancy in winter. In a cooler climate, lemon balm will often die right back to soil level. This is quite normal. Even plants need their sleep.

Do nothing. Just give it enough water over winter to keep the soil just barely moist.

We've had 5 frosts so far this fall at 31-32 degrees and my Lemon Balm isn't really bothered until a good hard freeze. It's still growing after the final harvest from about a month ago. How long ago was it transplanted to the ground from the pot? Was it cut back some before you stuck it in the ground? Watered in well? If it was say within the last 2 weeks it probably hasn't re-established and you are seeing the transplant shock.

Vera

I have to agree with Daisy. Lemon Balm starts looking pretty ratsy this time of year because the plant is pulling back to the roots and getting ready for the long winter sleep.

FataMorgana

Dunno then....mine still looks good after 5 frosts at 32-31 degrees since September 20th...the lastest frost occurred on October 5th. Here's a few pics from today....