What happens to Bitcoin wallet / Bitcoins if PC is stolen
Accessing to (your) bitcoin requires a private key.
This is kind a long number password to access the bitcoin on the network called blockchain. 

All bitcoin are saved in this (online) network.
A private key looks like this:
5Kb8kLf9zgWQnogidDA76MzPL6TsZZY36hWXMssSzNydYXYB9KF

Important thing is you save your private key on different places in case you need it back again. One good practice is print it on a paper with QR code. This is called Cold Storage. This is your backup.

Don't show this private key to anyone else and save a copy somewhere else than your home. 

Act like a bank, because with bitcoin you are own bank.
When your PC is gone, you take the private key and setup a new wallet on a device. Amount of the bitcoin will be exactly the same as it was left last time, because it is saved on the blockchain. 

Read more here
https://en.bitcoin.it/wiki/Private_key