How to store cryptocurrencies safely?
I agree this may be a duplicate question, but fundamentally you probably need to understand what is going on to understand what is at risk.

The blockchain is where the record of of your bitcoins are kept - they are not "on your computer".

What is "on your computer" is a set of keys which allow you to "approve" a transaction to move Bitcoins from a specified address.

There are two keys:
Private Key
Public key.

The private key is used to generate the public key, so as long as you have the private key, then you have the ability to:
a) create a public key
b) sign transactions

You cannot derive the private key from the public key - that would break all cryptography!

You do not have to do this with the Bitcoin QT client - there are other alternatives, Multibit being a good alternative as it is very quick to syncronise, and you can upload private keys easily.

If you keep only one copy of that private key - then you may have a problem if it gets lost or destroyed.

Therefore you should keep multiple copies.

To answer your question specifically - if you do not copy the file correctly you are screwed. If your computer is hacked, you are potentially screwed.

Checkout the link in the comment from @frisco for more information.