The private keys for your coin addresses are stored in the wallet.dat file.
These keys can not be reproduced.
If your computer/hard drive crashes and the wallet.dat file gets corrupt - or you delete that file,
your coins are gone forever, with no possible way of retrieving them.
seriously, make backups.
Most wallets have the option to make a backup built in.
For those that don't, or you want to know how to do it manually:
I use a thumb drive, and have folder for each of my coins, then I can put
the wallet.dat in a folder without having to rename it
Copy/Paste the addresses into notepad, or some other text editor
to get your private key, type
the wallet will print your private key
Copy/Paste the private key into your text editor.
Now do the same for each address you have.
You can save the text file of your private keys on the thumb drive with the wallet.dat file
You wallet may -or- may not scan the
blockchain automatically after you
if you have a block of 100 coins, and you send 25 coins somewhere,
your wallet has to use the whole block of 100 coins as an "input"
The remaining 75 (minus txfees) will be sent back to your wallet -
some times using a new address for the change.
This new address will not be in your backed up wallet.dat file
So - as you can imagine, you need to make "fresh" backups from time to time.
- and also dump the privite key of any new addresses.