I see a lot of assumptions being made here...
Are both eyes closed?
If only one is closed it is likely that the tortoise has gotten something in his eye. Oddly enough, my star tortoise just had something similar occur after getting coconut coir in his eye. I soaked several times a day and gently (very, very gently) swabbed the eye with terramycin twice a day. It took about three days for his eye to open completely and the swelling to go down.