Its stem has lain on the soil and looks weak. The first leaves are there but appear wilted.
One reason for this is a fungus infection called "damping off". This happens when the soil surface is too cool for rapid germination and also harbours a lot of fungal spores. The slow growth of the seedlings allows the fungal spores to penetrate the stem tissues and cut them off at soil level. To eliminate this possibility ensure that you do not use old soil that was used previously to germinate seeds or alternatively heat the soil or disinfect it before re-use, do not over-water the soil, and to germinate at the optimum temperature which allows the seedlings to be up and away before the fungus has a chance to become established.
Looking closely at the stems of tomato plants you will see that they are very hairy. This means a lot of soft tissue for the spores to become attached to the plant and remain viable.