Thanks for your question
You will need to phone the tax credit office to advise them of
1) The fact you are hospitalised
2) Will be out of the UK for longer than anticipated
3) Due to hospitalisation will be late with your tax credit renewal.
You will need to ring +44 2890 538 192 and usually these telephone numbers are just manned Monday to Friday from 8am to 8pm, but you could try today between 8am and 4pm.
They will advise how this change of circumstance affects your claim.
Is there not a way of working back from the amount received in tax credits to determine an amount previously declared?
Why would you need to do this, as the renewal is to declare what was received in income from 06/04/2012 to 05/04/2013, which will be fact, and not an estimate on the tax year 2011/2012 and the income received then??
But in answer to your question, tax credits are made up of various factors, children and their ages, any childcare, the working tax component and the child tax credit component, so no, this is not possible to work backwards.