I have an older model window a/c unit. It has been working fine. Yeaterday it froze up. What causes a window unit to freze up?

Dirty coils/dirty filter
Cool outside temperatures at night
Low on refrigerant charge

On the front of the unit is a gril that the return air goes through into the unit. Behind this is a filter, if this filter is dirty the unit will freeze over. If the unit is frozen over it is hard to get this filter off without tearing it up so let it defrost before you try. If there is not a filter then the coil itself is probably very dirty and needs cleaning. Plus you need to get a filter so this will not happen again.