Help: seedling stems dying off.

[ju1234]

First time trying hydroponics. I germinated various seeds in coconut coir + vermaculite. It was done indoor under a light. The seeds germinated fine, the seedling stems growing long (not too long) before they were put outside in day light and later in sun. Many of the seedling stems gradually dried up with leaves still looking good and then seedling just fell over and the whole plant just falls over(only one small real leaves). What might be causing it? Solution? Thanks.