What are the chances that I lost my bitcoins?
The only way to lose bitcoins is send them to some other address, have somebody steal your private keys and send them someplace else, or to lose the private keys.
As long as you backup the wallet properly and don't get hacked (or equivlent like getting a bitcoin stealing virus), you won't lose the coins. It is absolutely critical to back up the wallet properly, and at regular intervals. (The exception is that certain wallet programs create wallets that only need to be backed up once. These are called "deterministic wallets". Currently the main Bitcoin-QT program does not support them, but in the future it will.)
At this time the default Bitcoin-QT system does not make backups as user-friendly as it really should be, so if you have questions, be sure to ask.