Your tree's leaf drop could be due to over or under watering, pests, poor soil, disease, or other cultural problems such as dry indoor heat or lack of sunshine. To address your immediate problem, your first step is to determine whether or not the tree is still alive. To do this, scape of a little bark near the base of the tree. If it is still green, it is probably still alive.

I recommend you remove the blooms as they will likely drop off anyway with no nutrients from leaves to support them. This will allow all the energy the roots can provide to go into producing leaves. Your tree may or may not push out new leaves. If you are using the correct soil, a pot with adequate drainage holes (they hate soggy soil), and proper watering, there is a chance the tree will revive. There are no guarantees.

Going forward, please see the information and links below as there is a lot to know about growing a lemon tree indoors--especially in a northern climate. Here are a few essentials:

1. Light. Make sure your trees get at least 6 hours of sunlight a day. You can do this by placing your tree by a large, sunny window. If you can, try to place your tree near an area that faces South. Rotate the pot every three weeks. This way, the entire tree gets time in the sunshine!

2. Water. Overwatering or under-watering your tree can harm fruit production. The soil should slightly dry in between waterings, but it should never be completely dry--aim for moist. Check on your soil once a week. If it feels dry to the touch 2 inches below the surface, it’s time for more water. Generally, Meyer Lemon Trees need water every one to two weeks. Never allow the roots to remain in standing water, which often happens when the water you apply drips into a saucer and is allowed to stay there. Your tree cannot survive wet roots and requires extremely good drainage.

Nutrients. To give your tree an extra boost, give it some citrus fertilizer! Give your tree two tablespoons of fertilizer (depending on the size of the tree, follow directions) 3-4 times per year--in early spring, early summer, and again in the late summer and in the fall. Space out your fertilizing by about 4-6 weeks.

Temperature. When inside, your furnace heat can dry out your tree.. Be careful not to place it near a vent. If your leaves start to dry, you can mist them daily with a spray bottle for extra humidity.Once it warms up, move your tree outside for a few hours each day in a shady or dappled area, gradually increasing the amount of time it spends outdoors, before letting it live outside all summer.

Pruning. Keep the tree wide and branched out. When you prune your tree in the early fall or early spring, look for branches that are growing straight upwards; as generally, these aren’t fruit-producing branches. Remove any damaged or crossing branches. Make your cuts at 45-degree angles facing upwards to promote new growth. Look for areas that block the sunlight from the center of the tree. Removing these branches will increase air circulation and the amount of sunlight that hits these branches, which will decrease your tree’s risk of mold and fungi.

https://extension.umn.edu/house-plants/growing-citrus-indoors