Well done for coming off the cannabis. With regard to work its a tricky one for you. It may be that they have asumed you are not returning. Much depends on how long you have been there. If it is a big company they may have an occupational health department. If so then call personnel first thing tomorrow and ask for an appointment. At least that way your reasons can remain confidential. If not then you need to contact your manager and come up with a good reason. People breakdown for many reasons so it doesnt have to be the true reason if you feel that it will go against you. Have you visited your gp - he can issue you with a cetificte and I am sure that if he is aware of your problems then he will backdate it for you.
I hope you do have good support. My 19 yr old son was drug using since last summer and gave the cannabis up a couple of months ago. He was pretty awful for a few weeks and now he is back to his normal non aggresive self. It was a very difficult time for all of my family which he now realises.
Be strong and proud of yourself for coming off the stuff.
Good luck.