Peugeot 306 gearbox problems, need help urgent?

hello, i have a 1996 peugeot 306 d turbo, i have a problem with my gear box, hers what happend.
I was driving the car as usual, i left it running with my mates in the car for about 5min, when i got back the car sounded like it was going to stall so i gave it a bit of gas then when i when to move off i had to force it into firts gear and it seemed like it was going to stall. i stopped the car depressed the clutch all the way down and tried all the gears, i had to force it into 1st and second but when i put it into 3rd it wouldent come out, so i forced it out and know it wont go back in even when im driving, all the gears seems more clunky and it just wont go into third. What i really need to know is it the gearbox or cluth thats falied. I got my car for £500 and its not had any major problems so far, it used to go into gear easliy, nice and smooth. i dont know of this might help but when i came back to the car my mate who was sitting in the front passenger seat had his leg over and was revving the engin so i thought maby he was grinding the gears with his leg. Please could you help me, my battery has now gone totally flat and i cant even jump start it, so i cant get it to local mechanic to have a look, i dont want to fork out for a new battery if the repair is going to be to expensive as its only a £500 car with 140000 on the clock.
Thank you