gearbox stuck in 4th gear?

hi there i was jus wondering if anyone cud tell me my bf has a renault clio 1.8 16v and the car is stuck in 4th gear and wont go in any other gears when car is started, wen car isnt started it wont go in 5th or 1st we have been told it mite of siezed up due to it mite not have any gearbox fluid in does this sound right to any ov you's