Having bad abdominal pains and rang my gp today so early but now they say they haven’t got any space…

Having bad abdominal pains and rang my gp today so early but now they say they haven’t got any space so can’t see me today what should I do? I also been trying to ring for days so I could get a letter from doctors sign me off night shift because I just can’t deal with it been so tired and drained all the time 😩 but still no luck