Hi all. I am new to the bike repair game and certainly need some help. I have just gone to the shed to get the bike out and found the rear tyre to be very flat i.e. almost off the rim. First thought was a puncture, but when I put the air compressor on, the air just rushes out around the rim. After a closer inspection, there is a round hole in the rim with nothing in it. The valve stem is in place however. Can someone please tell me what has happened? Is it a missing rim lock stem? Can I fix it or do I need help? Please help. Thanks Kev.