While starting plants from seeds the developing seedlings will grow in the direction of the light. This is called Phototropism.
If the light source for your growing seedlings is not strong enough or does not contain the proper spectrum of light then the cells of the developing seedling can develop thin cell walls and elongate. Thus, results in tall spindly seedlings that can not support their own weight.
Once you have stretch/leggy seedlings there are some things you can do to address the issue
For some plants such as Solanaceous vegetables Tomatoes, peppers, Pumpkin, Zucchini, Cucumber, Bitter Melon, Nasturtium, marigolds and Eggplants they can generate new roots from both the existing transplant root system and also from stem that are buried during repotting. Roots generated on the stems are called adventitious roots and in solanaceous transplants they can grow at any place along the stem above the root system.
Plants such as cucumbers, pumpkins, squash and watermelons will only generate adventitious roots at above-ground nodes where the leaves join the stems.
With seedlings like squash, cucumber, pumpkin this will limit what seedlings could take advantage of this condition since typically no nodal tissue will be in contact with soils at transplanting.
A good rule of thumb to identify if a plant will form adventitious roots is if you can take a cutting from the plant without roots it will form roots then it is a plant that will create adventitious roots when the leggy stem is transplanted deeper into the soil.
The other option for seedlings that will not form adventitious roots is to burry the elongated stems in the soil when potting up or transplanting.
This method is not with out risk. All plants have surface level roots that are responsible for taking up water and nutrients to the rest of the plant; if the root ball is planted too deep, these roots can suffocate or rot especially buried in damp soil.
The other issue us that these elongated stems are very weak and can be damaged such that the seedlings might not survive.
For seedlings that will not form adventitious roots the best option is to start over and change the lighting to ensure that it is strong enough and of the proper spectrum.