Sometimes, seedlings need a little help to arise. If you have to take action, give special care because it is insanely fragile.
Keep your seeds in a hermetic box in the fridge. That way, seeds are dormant and remain alive for years. Some strains ask for a dormancy period before germinating. So if your seeds are fresh but don’t germinate, put them in your fridge for 1 week, and then try to germinate them.
If the seed has started to germinate and suddenly stopped, it’s probably a fungus or bacterial problem. Same if the seedling, after 1 week or so starts to mold and die. It’s the damping-off. When you face this problem, it’s already too late, you can’t do much to save your seedlings.
The only solution is to prevent it. Before starting the seedlings, clean everything you can, including the room where you grow. Everything has to be clean, your hands too !
Give special care to the origin of your substrate and it’s quality, very often problems come from bad soils from bad retailers.
To stake all the odds in your favour, you can use beneficial fungus. The most common is Trichoderma harzianum but many different species exist and make a symbiotic relation with the plant, giving it an extra strength and hardiness.
When the seedling lacks light, it will start to stretch to look for more light. This stretching can make the stem very weak.
Three ways to deal with it :
Bend the seedling super gently, and recover it with soil.
Repot and bury the whole stem, letting only the cotyledon out of the soil.
Use a little stake to support the seedling.