Maintained light works fine as a normal light, the grey LED is on but when I test the emergency light, it doesn't work. What could be the problem?

I presume you mean green light? If the light does not stay on when the mains is removed (You said it is a maintained light) that means the batteries are no longer holding their charge and should be changed. Safelincs can supply replacement batteries Click here

That said if the light has a fluorescent tube you may want to consider changing it to LED as there is not that much differance in price Click here

Agreed, sounds like the batteries!