Hi there. A closed eye is usually a sign of a sinus infection- the sinus surrounds the eye, when it gets infected and swollen it results in the eye closing.
There are a few infections that can result in a sinus infection, but a very common one is called 'Mycoplasmosis' which causes respiratory signs, often starting with closed eyes, a foamy discharge from the eyes, and then progressing to open mouthed breathing and in some cases death.
If caught early enough it can be treated with an injection of a drug called 'Tylosin'(Tylan) quite effectively.
I would look out for further respiratory signs in this bird and if they progress then inject all your birds with Tylan.
If the infection stays only within the sinus of the eye, then oral antibiotics will still be required to clear it, but something such as Doxycycline (ornimed, Ornicure) should help clear this.