I get this pretty frequently when starting seedlings. What's probably holding the cotyledons together is a protective sheath that acts as a second waxy barrier between the shell and the ungerminated sprout. Sometimes this gets stuck to the sprout just after dropping the seed shell. Just take it between your fingernails, and GENTLY slide it off of the leaves. As long as your not overly forceful, you shouldn't damage the sprout. Try not to move the stem a whole lot while you're doing this. Once the cotyledons spread apart, it should resume growth as normal.