If your rabbit cannot open its eyes, then you will need to act. There are several different reasons for rabbits not being able to open their eyes, but discharge and injury tend to be the most common culprits. If you are sure that it’s not due to any injury, and that it’s just a one off thing caused by a particularly dusty day, then you can try using a moist cloth to remove any gunk from your bunnies eye.
This Chinchilla Rabbit has nice, healthy eyes
It may be that your rabbit is unable to open its eye due to a buildup of substances secreted by the eye itself. If this is the case, get moist, clean cloth and sit your rabbit on your lap. Gently place the moist cloth on your pet’s eye. Make sure that you don't apply any pressure, just gently dab the eye so that the water can weaken the substance a little.
After a few seconds, gently wipe the closed eye once or twice, again without applying any pressure. If after doing this the eye still doesn’t open, then repeat the steps again. If after four or five repeats you’re still having no luck, it may be a good idea to take your pet to the vets.