Dear Lisa,
Lemons do not like being pruned,unless you paint over the wound with either expensive wound dressing ... or plain old (cheap)water-base acrylic paint (type you wash the brush clean in water).

They also do not like alkaline soils, having the trunk base covered, and get sub-burnt (particularly stems/branches) if in direct sunlight - on paving or by brick walls/asbestos, tin, iron, brick or rock-wool fencing ... ie. materials that store and reflect heat.

From the looks of it your tree first was stressed from lack of potash (sprinkle some charcoal or ash around the top of the pot and wash it in - bucket of burnt "two or three pages> newspaper, mixed with water will do the trick; as well as being unhappy about the soil acidity, and sunburnt (leaves and dead stems), or dying-back from previous pruning.

I am aware Peter Cunnel does not think you need to seal tree/plant wounds, however he is wrong in many cases; Citrus trees, oaks and mulberries being amongst those where post-prunning die-back can be serious/fatal ... but actually like el-cheapo acrylic paint - dabbed on wounds to seal in moisture and keep disease/bugs/ants out.

For the trunk base, think letter W in cross section - where the stem/trunk is the middle and pot edge = two top outer top legs (of 'W'). Rather than dig, just use a dry paint brush to gently clear the soil away until 5~10 mm from where the roots start, then make a dip-bowl effect about 25mm out and curving up to the pot edge. Purpose is so water soaks in well clear of the steam/trunk, and keeps the stem/trunk free of residual moisture and/or fungus growths.

Soil acidity is easy in this case (yours). Run down the shop and buy a couple of lemons or oranges, use former on chips/whatever or eat latter, keeping the skins/rinds. Put the rinds in the sun to dry, then put three or four dried half rinds in the dip/bowl of your pot, just out from the tree stem. As the skin rots/degrades, it balances the soil to the citrus tree's preferred measure

Sunburn. For the time being move the pot/tree out of the sun and its current position, if possible putting it on grass to avoid heat reflecting up from the paving and back from the brick wall.

When it is recovered and bushy again, put it back but place sacking, cardboard, carpet or a dead newspaper under the potbase to insulate against heat/reflected heat.

It should be better/recovered in a month or two. Basically you've cooked it, then compounded the problems by pruning it and leaving the wounds open to air.

Follow the instructions given and it will recover, then be so happy it might start jumping about and you'll probably find it wants to go 'walkies'. There again, maybe not, but it will be happy.

Cheers,

Rick.

ps. If pruning in the future, wash the cutters in a dettol or eucalyptus-pineOclean solution before and after ... to avoid infecting your plants/trees with disease.