This could be a number of things. Can you tell us a bit about your tank?
Size of Tank
Age of Tank
Stock (what fish, how many, and how long have they been there)
Any aggression that you have noticed between inhabitants?
pH
Nitrate
Ammonia
Nitrite
Temp
PO4 (if available)
Your maintenance schedule (i.e. how many water changes, what size, how often?)
What kind of filter?
Is the tetra's eye completely gone, or is it just bulging out of the socket?
If it is pop eye, which is a disease, it would be caused by water conditions. Giving us the above parameters will help determine a fix. If the eye is completely missing, it might be an aggression issue, which we can help identify once we know the tank size and stock.