Harry,
Firstly, i'm assuming your handbrake is off. if it isn't, you will need to put it in the down position.
If you know how to adjust the handbrake from inside the car, you should make it as slack as possible to try to get the drum back on.
If this doesn;t help, you will most likely need to take out the adjusting mechanism that looks like a strange bolt that goes between the two shoes and wind it in to its shortest length. then you should be able to put the shoes back into thei normal position and put the drum back on.
if you are wanting to drive the car with effective brakes later, you will need to adjust the brakes once the drum is back on.
If you don't know how to do all of this, i would suggest googling the subject, or searching on youtube.
best of luck.
-dave