Hello,
No you can't join back from the point you left. If you had left it and it's gone then you can't join in middle.
You have to repeat again. It really depends on the reason why you were detained. If there is any serious health issue then you can join with the permission of college authorities and if you have missed one semester then you have to give back paper for one whole semester in next odd term.
So better to go and discuss it will the college authorities and ask them to guide you. If they say no at any Ground then you have to repeat it again.
Hope it helps!